diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index d128167055f0786710f12a8b72a843a3f54c44c5..13a5154d902c4179b46ecc822fc59da2b02a9783 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1183,10 +1183,10 @@ function template_preprocess_maintenance_task_list(&$variables) {
  * This function is called for theme hooks implemented as templates only, not
  * for theme hooks implemented as functions. This preprocess function is the
  * first in the sequence of preprocessing functions that are called when
- * preparing variables for a template. See _theme() for more details about the
- * full sequence.
+ * preparing variables for a template.
  *
- * @see _theme()
+ * See the @link themeable Default theme implementations topic @endlink for
+ * details.
  */
 function template_preprocess(&$variables, $hook, $info) {
   // Tell all templates where they are located.
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 5e450ab851e0e1925b8f33d89ef1e9ed64ff0315..bb8970737f4bef87a44f8687e24ec29761481b20 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -298,11 +298,13 @@ public function getBaseHook($hook) {
    * for base hooks (e.g., 'block__node' for the base hook 'block') need to be
    * determined based on the full registry and classified as 'base hook'.
    *
-   * @see _theme()
-   * @see hook_theme_registry_alter()
+   * See the @link themeable Default theme implementations topic @endlink for
+   * details.
    *
    * @return \Drupal\Core\Utility\ThemeRegistry
    *   The build theme registry.
+   *
+   * @see hook_theme_registry_alter()
    */
   protected function build() {
     $cache = array();
diff --git a/core/lib/Drupal/Core/Theme/ThemeManagerInterface.php b/core/lib/Drupal/Core/Theme/ThemeManagerInterface.php
index fdc633438b6555bc0deea4b545e28d75cc83f04c..ebbfcb5fb723e3c5ad9313ae85e232f934d31b89 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManagerInterface.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManagerInterface.php
@@ -18,6 +18,9 @@ interface ThemeManagerInterface {
   /**
    * Generates themed output.
    *
+   * See the @link themeable Default theme implementations topic @endlink for
+   * details.
+   *
    * @param string $hook
    *   The name of the theme hook to call.
    * @param array $variables
@@ -25,8 +28,6 @@ interface ThemeManagerInterface {
    *
    * @return string
    *   The rendered output.
-   *
-   * @see _theme
    */
   public function render($hook, array $variables);
 
diff --git a/core/modules/block/block.api.php b/core/modules/block/block.api.php
index 7f5269f521a67fff4fe8d849c2fa6d7b4a1f5ee1..afd2beff907553917b69ec83c4f2c3a6d61dad75 100644
--- a/core/modules/block/block.api.php
+++ b/core/modules/block/block.api.php
@@ -72,8 +72,8 @@
  * If the module wishes to act on the rendered HTML of the block rather than
  * the structured content array, it may use this hook to add a #post_render
  * callback. Alternatively, it could also implement hook_preprocess_HOOK() for
- * block.html.twig. See drupal_render() and _theme() documentation respectively
- * for details.
+ * block.html.twig. See drupal_render() documentation or the
+ * @link themeable Default theme implementations topic @endlink for details.
  *
  * In addition to hook_block_view_alter(), which is called for all blocks, there
  * is hook_block_view_BASE_BLOCK_ID_alter(), which can be used to target a
diff --git a/core/modules/system/entity.api.php b/core/modules/system/entity.api.php
index ee4d4be1661a2d6a966f7c567da2e237f840c317..95e582236c9e3de18cefbcccf198ff5ab122f7aa 100644
--- a/core/modules/system/entity.api.php
+++ b/core/modules/system/entity.api.php
@@ -1327,7 +1327,9 @@ function hook_ENTITY_TYPE_view(array &$build, \Drupal\Core\Entity\EntityInterfac
  * structured content array, it may use this hook to add a #post_render
  * callback. Alternatively, it could also implement hook_preprocess_HOOK() for
  * the particular entity type template, if there is one (e.g., node.html.twig).
- * See drupal_render() and _theme() for details.
+ *
+ * See the @link themeable Default theme implementations topic @endlink and
+ * drupal_render() for details.
  *
  * @param array &$build
  *   A renderable array representing the entity content.
@@ -1337,10 +1339,10 @@ function hook_ENTITY_TYPE_view(array &$build, \Drupal\Core\Entity\EntityInterfac
  *   The entity view display holding the display options configured for the
  *   entity components.
  *
+ * @ingroup entity_crud
+ *
  * @see hook_entity_view()
  * @see hook_ENTITY_TYPE_view_alter()
- *
- * @ingroup entity_crud
  */
 function hook_entity_view_alter(array &$build, Drupal\Core\Entity\EntityInterface $entity, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display) {
   if ($build['#view_mode'] == 'full' && isset($build['an_additional_field'])) {
@@ -1363,7 +1365,9 @@ function hook_entity_view_alter(array &$build, Drupal\Core\Entity\EntityInterfac
  * structured content array, it may use this hook to add a #post_render
  * callback. Alternatively, it could also implement hook_preprocess_HOOK() for
  * the particular entity type template, if there is one (e.g., node.html.twig).
- * See drupal_render() and _theme() for details.
+ *
+ * See the @link themeable Default theme implementations topic @endlink and
+ * drupal_render() for details.
  *
  * @param array &$build
  *   A renderable array representing the entity content.
@@ -1373,10 +1377,10 @@ function hook_entity_view_alter(array &$build, Drupal\Core\Entity\EntityInterfac
  *   The entity view display holding the display options configured for the
  *   entity components.
  *
+ * @ingroup entity_crud
+ *
  * @see hook_ENTITY_TYPE_view()
  * @see hook_entity_view_alter()
- *
- * @ingroup entity_crud
  */
 function hook_ENTITY_TYPE_view_alter(array &$build, Drupal\Core\Entity\EntityInterface $entity, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display) {
   if ($build['#view_mode'] == 'full' && isset($build['an_additional_field'])) {
diff --git a/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
index eef4f5c360d1a1b7f6c8db8e9292ac30c299090d..8eae7f5eed7085e322d943750cd0dae59b56a629 100644
--- a/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -970,12 +970,8 @@ function hook_page_bottom(array &$page_bottom) {
 /**
  * Register a module or theme's theme implementations.
  *
- * The implementations declared by this hook have several purposes:
- * - They can specify how a particular render array is to be rendered as HTML.
- *   This is usually the case if the theme function is assigned to the render
- *   array's #theme property.
- * - They can return HTML for default calls to _theme().
- * - They can return HTML for calls to _theme() for a theme suggestion.
+ * The implementations declared by this hook specify how a particular render
+ * array is to be rendered as HTML.
  *
  * @param array $existing
  *   An array of existing implementations that may be used for override
@@ -1001,19 +997,18 @@ function hook_page_bottom(array &$page_bottom) {
  *
  * @return array
  *   An associative array of information about theme implementations. The keys
- *   on the outer array are known as "theme hooks". For simple theme
- *   implementations for regular calls to _theme(), the theme hook is the first
- *   argument. For theme suggestions, instead of the array key being the base
- *   theme hook, the key is a theme suggestion name with the format
- *   'base_hook_name__sub_hook_name'. For render elements, the key is the
- *   machine name of the render element. The array values are themselves arrays
- *   containing information about the theme hook and its implementation. Each
- *   information array must contain either a 'variables' element (for _theme()
- *   calls) or a 'render element' element (for render elements), but not both.
+ *   on the outer array are known as "theme hooks". For theme suggestions,
+ *   instead of the array key being the base theme hook, the key is a theme
+ *   suggestion name with the format 'base_hook_name__sub_hook_name'.
+ *   For render elements, the key is the machine name of the render element.
+ *   The array values are themselves arrays containing information about the
+ *   theme hook and its implementation. Each information array must contain
+ *   either a 'variables' element (for using a #theme element) or a
+ *   'render element' element (for render elements), but not both.
  *   The following elements may be part of each information array:
- *   - variables: Used for _theme() call items only: an array of variables,
+ *   - variables: Only used for #theme in render array: an array of variables,
  *     where the array keys are the names of the variables, and the array
- *     values are the default values if they are not passed into _theme().
+ *     values are the default values if they are not given in the render array.
  *     Template implementations receive each array key as a variable in the
  *     template file (so they must be legal PHP/Twig variable names). Function
  *     implementations are passed the variables in a single $variables function
@@ -1041,7 +1036,7 @@ function hook_page_bottom(array &$page_bottom) {
  *   - function: If specified, this will be the function name to invoke for
  *     this implementation. If neither 'template' nor 'function' are specified,
  *     a default template name will be assumed. See above for more details.
- *   - base hook: Used for _theme() suggestions only: the base theme hook name.
+ *   - base hook: Used for theme suggestions only: the base theme hook name.
  *     Instead of this suggestion's implementation being used directly, the base
  *     hook will be invoked with this implementation as its first suggestion.
  *     The base hook's files will be included and the base hook's preprocess
@@ -1051,14 +1046,17 @@ function hook_page_bottom(array &$page_bottom) {
  *     suggestion may be used in place of this suggestion. If after
  *     hook_theme_suggestions_HOOK() this suggestion remains the first
  *     suggestion, then this suggestion's function or template will be used to
- *     generate the output for _theme().
+ *     generate the rendered output.
  *   - pattern: A regular expression pattern to be used to allow this theme
  *     implementation to have a dynamic name. The convention is to use __ to
  *     differentiate the dynamic portion of the theme. For example, to allow
  *     forums to be themed individually, the pattern might be: 'forum__'. Then,
- *     when the forum is themed, call:
+ *     when the forum is rendered, following render array can be used:
  *     @code
- *     _theme(array('forum__' . $tid, 'forum'), $forum)
+ *     $render_array = array(
+ *       '#theme' => array('forum__' . $tid, 'forum'),
+ *       '#forum' => $forum,
+ *     );
  *     @endcode
  *   - preprocess functions: A list of functions used to preprocess this data.
  *     Ordinarily this won't be used; it's automatically filled in. By default,
@@ -1078,6 +1076,7 @@ function hook_page_bottom(array &$page_bottom) {
  *   - theme path: (automatically derived) The directory path of the theme or
  *     module, so that it doesn't need to be looked up.
  *
+ * @see themeable
  * @see hook_theme_registry_alter()
  */
 function hook_theme($existing, $type, $theme, $path) {