Inline documentation for hooks in wp-admin/includes/image.php.

Props theorboman, kpdesign.
Fixes #25832.

Built from https://develop.svn.wordpress.org/trunk@27202


git-svn-id: http://core.svn.wordpress.org/trunk@27059 1a063a9b-81f0-0310-95a4-ce76da25c4cd