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.

Built from https://develop.svn.wordpress.org/trunk@30758


git-svn-id: http://core.svn.wordpress.org/trunk@30748 1a063a9b-81f0-0310-95a4-ce76da25c4cd