Docs: Improve documentation for admin menu functions:

* `get_admin_page_parent()`
* `get_admin_page_title()`
* `get_plugin_page_hook()`
* `get_plugin_page_hookname()`
* `user_can_access_admin_page()`

Add missing descriptions and `@since` tags.

Props subrataemfluence, kennithnichol, stevenlinx, SergeyBiryukov.
Fixes #44253, #49067.
Built from https://develop.svn.wordpress.org/trunk@47006


git-svn-id: https://core.svn.wordpress.org/trunk@46806 1a063a9b-81f0-0310-95a4-ce76da25c4cd