Added explanatory note on CSRF failure page for the case of a missing Referer header.

This is intended to help power users who have disabled Referer headers, or
installed add-ons which have done so, and to help web site administrators
with debugging, since this problem will be browser specific and not a
programming error.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@13680 bcc190cf-cafb-0310-a4f2-bffc1f526a37