Fixed #7942 -- Added a note to the admin ordering option documentation about...

Fixed #7942 -- Added a note to the admin ordering option documentation about the admin only respecting the first element. Thanks mwdiers for the report and MattBowen for the original patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8210 bcc190cf-cafb-0310-a4f2-bffc1f526a37