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.

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git-svn-id: https://core.svn.wordpress.org/trunk@26962 1a063a9b-81f0-0310-95a4-ce76da25c4cd