Fixed #12130 - documented need for csrf_protect on views that don't accept POST

Includes:
   
 * proper documentation for csrf_protect
 * notes in comments app.
 * specific upgrade notes for comments app

Thanks to carljm for report and debugging.




git-svn-id: http://code.djangoproject.com/svn/django/trunk@11711 bcc190cf-cafb-0310-a4f2-bffc1f526a37