Inline documentation for hooks in wp-admin/media-upload.php.

Props piontkowski for the initial patch.
Fixes #26075.

Built from https://develop.svn.wordpress.org/trunk@26917


git-svn-id: http://core.svn.wordpress.org/trunk@26798 1a063a9b-81f0-0310-95a4-ce76da25c4cd