Inline documentation for hooks in wp-admin/admin-ajax.php.

Props nullvariable for the initial patch.
Fixes #25229.

Built from https://develop.svn.wordpress.org/trunk@25538


git-svn-id: http://core.svn.wordpress.org/trunk@25458 1a063a9b-81f0-0310-95a4-ce76da25c4cd