Fixed django#16168 - Added note regarding type requirements when overridng ModelForm fields.

Thanks Pieter Swinkels for the patch.

Author: timgraham

docs/topics/forms/modelforms.txt

@@ -437,6 +437,10 @@ parameter when declaring the form field::
  class Meta:
  model = Article
 
+ You must ensure that the type of the form field can be used to set the
+ contents of the corresponding model field. When they are not compatible,
+ you will get a ``ValueError`` as no implicit conversion takes place.
+
  See the :doc:`form field documentation </ref/forms/fields>` for more information
  on fields and their arguments.