Fixed #8620 -- Updated the Form metaclass to support excluding fields by shadowing them.

docs/topics/forms/modelforms.txt

@@ -651,6 +651,18 @@ There are a couple of things to note, however.
   because these classes rely on different metaclasses and a class can only have
   one metaclass.
 
+.. versionadded:: 1.7
+
+* It's possible to opt-out from a ``Field`` inherited from a parent class by
+  shadowing it. While any non-``Field`` value works for this purpose, it's
+  recommended to use ``None`` to make it explicit that a field is being
+  nullified.
+
+  You can only use this technique to opt out from a field defined declaratively
+  by a parent class; it won't prevent the ``ModelForm`` metaclass from generating
+  a default field. To opt-out from default fields, see
+  :ref:`controlling-fields-with-fields-and-exclude`.
+
 .. _modelforms-factory:
 
 ModelForm factory function
@@ -749,6 +761,8 @@ instances of the model, you can specify an empty QuerySet::
    >>> AuthorFormSet(queryset=Author.objects.none())
 
 
+.. _controlling-fields-with-fields-and-exclude:
+
 Controlling which fields are used with ``fields`` and ``exclude``
 -----------------------------------------------------------------