Issue #2408265 by wadmiraal: Update hook_theme_registry_alter to not reference removed user.pages.inc

Issue #2430909 by anwar_max, wadmiraal, harshil.maradiya, joshi.rohit100, ClientGuy: hook_theme_suggestions() hook_theme_suggestions_HOOK() documentation is incorrect about how the hook is invoked

Issue #2373741 by Wim Leers, hussainweb: Move bulk of render pipeline documentation to d.o handbooks

Issue #2444211 by Wim Leers, jhodgdon: Document cacheability of render arrays, and the considerations to use when generating render arrays

Issue #2396553 by nripeshtrivedi, fullerja: field.html.twig documentation on template suggestions is incorrect

Issue #2368797 by Wim Leers, dawehner, rteijeiro: Optimize ajaxPageState to keep Drupal 8 sites fast on high-latency networks, prevent CSS/JS aggregation from taking down sites and use HTTP GET for AJAX requests

Issue #2358981 by tadityar, tstoeckler, larowlan, mpdonadio, Devin Carlson: Provide a mechanism for dynamic library declarations

Issue #2382497 by er.pushpinderrana: The first parameter of the hook function hook_page_bottom is wrong in the documentation

Issue #2367285 by cilefen: Fixed Function doc typo for hook_page_attachments_alter() in theme.api.php.

Issue #2307859 by rpayanm, dankh, shumer: Move theme-render hooks from system.api.php to theme.api.php

Issue #2350949 by Wim Leers: Add hook_page_attachments(_alter)() and deprecate hook_page_build/alter().

Issue #2340667 by YesCT, Pedro Lozano, alexpott, martin107: Protect Drupal\Core\Language\Language::id, and use getId().

Issue #2246675 by rpayanm, johnstorey, holly.ross.drupal | mgbellaire: Remove all unnecessary 'template' lines in hook_theme() declarations.

Issue #2346369 by Wim Leers: Support special '#attached' variable for attaching assets in preprocess functions.

Issue #1332068 by Cottser, pixelmord | tim.plunkett: ENGINE_render_template() and ENGINE_extension() are undocumented.

Issue #2326093 by astrocling | jhodgdon: Need new section about Twig added to theme system overview.

Issue #2312069 by er.pushpinderrana, dawehner: Fix formatting problem on Default theme implementations topic

Issue #2061977 by InternetDevels, kim.pepper, ianthomas_uk, herom, rhm50, naveenvalecha, andypost,  mandar.harkare, sergeypavlenko, sidharthap, SIz, tkuldeep17: Replace user_access() calls with ->hasPermission() in all core modules except user.

Revert "Issue #2061977 by InternetDevels, kim.pepper, ianthomas_uk, herom: Replace user_access() calls with ->hasPermission() in all core modules except user."

Fixing commit credit.

This reverts commit 25afa810.