Fixed #12812 -- added warning about cyclic imports in contrib.comments. Thanks...

Fixed #12812 -- added warning about cyclic imports in contrib.comments. Thanks to Beetle_B, bjourne and philgyford for the reports, and Russ for the wording.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14810 bcc190cf-cafb-0310-a4f2-bffc1f526a37