Inline documentation cleanup for 4.0 audit.

* Fill out inline documentation for the `__call()` magic method added in [28507]
* Inline documentation tweaks for `get_provider()`, added in [28728]
* Inline documentation tweaks for `_add_provider_early()`, added in [28846]
* `@access` tag added for `_remove_provider_early()`, added in [28846]

See #22234 and #28885.

Built from https://develop.svn.wordpress.org/trunk@29155


git-svn-id: http://core.svn.wordpress.org/trunk@28939 1a063a9b-81f0-0310-95a4-ce76da25c4cd