Inline documentation for hooks in wp-admin/includes/class-wp-ms-themes-list-table.php.

Also marks two hooks in wp-admin/includes/class-wp-themes-list-table.php as duplicates.

Props ShinichiN, kpdesign, DrewAPicture.
Fixes #25608.


git-svn-id: https://develop.svn.wordpress.org/trunk@27090 602fd350-edb4-49c9-b593-d223f7449a82