Issue #2392221 by er.pushpinderrana, ClientGuy, zealfire: install_run_task() and install_tasks() don't document the task structure

Issue #2151469 by rpayanm, lokapujya, JeroenT, alansaviolobo, Richard Damon, pflame, ianthomas_uk: Clean-up usage of deprecated list_themes() and _system_rebuild_theme_data() in favor of theme_handler service.

Issue #1993452 by Gábor Hojtsy, YesCT, bannorb, rvilar, Pancho, Sutharsan, Albert Volkman: Fix confusing UX by merging "Translate configuration" into "Finish translations" task

Issue #2342023 by rpayanm, Matt V.: documentation references hook_disable and hook_modules_disabled, but they've been removed