Fixed #24295 -- Allowed ModelForm meta to specify form field classes.

Thanks Carl Meyer and Markus Holtermann for the reviews.

docs/topics/forms/modelforms.txt

@@ -475,9 +475,8 @@ Overriding the default fields
 
 The default field types, as described in the `Field types`_ table above, are
 sensible defaults. If you have a ``DateField`` in your model, chances are you'd
-want that to be represented as a ``DateField`` in your form. But
-``ModelForm`` gives you the flexibility of changing the form field type and
-widget for a given model field.
+want that to be represented as a ``DateField`` in your form. But ``ModelForm``
+gives you the flexibility of changing the form field for a given model.
 
 To specify a custom widget for a field, use the ``widgets`` attribute of the
 inner ``Meta`` class. This should be a dictionary mapping field names to widget
@@ -525,9 +524,8 @@ the ``name`` field::
                 },
             }
 
-Finally, if you want complete control over of a field -- including its type,
-validators, etc. -- you can do this by declaratively specifying fields like you
-would in a regular ``Form``.
+You can also specify ``field_classes`` to customize the type of fields
+instantiated by the form.
 
 For example, if you wanted to use ``MySlugFormField`` for the ``slug``
 field, you could do the following::
@@ -536,13 +534,18 @@ field, you could do the following::
     from myapp.models import Article
 
     class ArticleForm(ModelForm):
-        slug = MySlugFormField()
-
         class Meta:
             model = Article
             fields = ['pub_date', 'headline', 'content', 'reporter', 'slug']
+            field_classes = {
+                'slug': MySlugFormField,
+            }
 
 
+Finally, if you want complete control over of a field -- including its type,
+validators, required, etc. -- you can do this by declaratively specifying
+fields like you would in a regular ``Form``.
+
 If you want to specify a field's validators, you can do so by defining
 the field declaratively and setting its ``validators`` parameter::
 
@@ -556,6 +559,10 @@ the field declaratively and setting its ``validators`` parameter::
             model = Article
             fields = ['pub_date', 'headline', 'content', 'reporter', 'slug']
 
+.. versionadded:: 1.9
+
+    The ``Meta.field_classes`` attribute was added.
+
 .. note::
 
     When you explicitly instantiate a form field like this, it is important to