Fixed #12643 -- Added link to the ModelForm documentation clarifying the...

Fixed #12643 -- Added link to the ModelForm documentation clarifying the special requirements of a clean() method on a modelform. Thanks to fgaudin for the report, and ptone for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13163 bcc190cf-cafb-0310-a4f2-bffc1f526a37