Add missing DocBlocks and parameter descriptions for a wide variety of properties and methods in `WP_Upgrader`, `Plugin_Upgrader`, `Theme_Upgrader`, `Language_Pack_Upgrader`, `Core_Upgrader`, and `File_upload_Upgrader`.

Props jdgrimes, DrewAPicture.

See #29086.


git-svn-id: https://develop.svn.wordpress.org/trunk@30758 602fd350-edb4-49c9-b593-d223f7449a82