Fixed #14885 - Clarified that ModelForm cleaning may not fully complete if the form is invalid.

Thanks Ben Sturmfels for the patch.

docs/topics/forms/modelforms.txt

@@ -200,7 +200,9 @@ The first time you call ``is_valid()`` or access the ``errors`` attribute of a
 well as :ref:`model validation <validating-objects>`. This has the side-effect
 of cleaning the model you pass to the ``ModelForm`` constructor. For instance,
 calling ``is_valid()`` on your form will convert any date fields on your model
-to actual date objects.
+to actual date objects. If form validation fails, only some of the updates
+may be applied. For this reason, you'll probably want to avoid reusing the
+model instance.
 
 
 The ``save()`` method