Add baseline doc blocks for all ajax handlers in ajax-actions.php.

Fixes #28200.

Built from https://develop.svn.wordpress.org/trunk@28355


git-svn-id: http://core.svn.wordpress.org/trunk@28183 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix mid-file section header style in wp-admin/includes/ajax-actions.php.

See #28200.

Built from https://develop.svn.wordpress.org/trunk@28356


git-svn-id: http://core.svn.wordpress.org/trunk@28184 1a063a9b-81f0-0310-95a4-ce76da25c4cd