Rename the `$action` hook in wp-admin/admin-post.php to `admin_post{$action}`.

Also, clarify documentation of priv vs nopriv prefixing of the hook name.

See #26869.


git-svn-id: https://develop.svn.wordpress.org/trunk@28351 602fd350-edb4-49c9-b593-d223f7449a82