Fixed #8857 -- Corrected ref in modelforms documentation and added ref to file...

Fixed #8857 -- Corrected ref in modelforms documentation and added ref to file upload documentation in form documentation. Thanks Kyle Fox and prairiedogg.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10837 bcc190cf-cafb-0310-a4f2-bffc1f526a37