Added note to docs/form_for_model.txt (from [6844]) explaining this is...

Added note to docs/form_for_model.txt (from [6844]) explaining this is deprecated in favor of ModelForms

git-svn-id: http://code.djangoproject.com/svn/django/trunk@6944 bcc190cf-cafb-0310-a4f2-bffc1f526a37