Fixed #19733 - deprecated ModelForms without 'fields' or 'exclude', and added '__all__' shortcut

This also updates all dependent functionality, including modelform_factory
 and modelformset_factory, and the generic views `ModelFormMixin`,
 `CreateView` and `UpdateView` which gain a new `fields` attribute.

docs/ref/contrib/admin/index.txt

@@ -335,6 +335,22 @@ subclass::
 
     For an example see the section `Adding custom validation to the admin`_.
 
+    .. admonition:: Note
+
+        .. versionchanged:: 1.6
+
+        If you define the ``Meta.model`` attribute on a
+        :class:`~django.forms.ModelForm`, you must also define the
+        ``Meta.fields`` attribute (or the ``Meta.exclude`` attribute). However,
+        since the admin has its own way of defining fields, the ``Meta.fields``
+        attribute will be ignored.
+
+        If the ``ModelForm`` is only going to be used for the admin, the easiest
+        solution is to omit the ``Meta.model`` attribute, since ``ModelAdmin``
+        will provide the correct model to use. Alternatively, you can set
+        ``fields = []`` in the ``Meta`` class to satisfy the validation on the
+        ``ModelForm``.
+
     .. admonition:: Note
 
         If your ``ModelForm`` and ``ModelAdmin`` both define an ``exclude``
@@ -1283,13 +1299,24 @@ templates used by the :class:`ModelAdmin` views:
     on the changelist page. To use a custom form, for example::
 
         class MyForm(forms.ModelForm):
-            class Meta:
-                model = MyModel
+            pass
 
         class MyModelAdmin(admin.ModelAdmin):
             def get_changelist_form(self, request, **kwargs):
                 return MyForm
 
+    .. admonition:: Note
+
+        .. versionchanged:: 1.6
+
+        If you define the ``Meta.model`` attribute on a
+        :class:`~django.forms.ModelForm`, you must also define the
+        ``Meta.fields`` attribute (or the ``Meta.exclude`` attribute). However,
+        ``ModelAdmin`` ignores this value, overriding it with the
+        :attr:`ModelAdmin.list_editable` attribute. The easiest solution is to
+        omit the ``Meta.model`` attribute, since ``ModelAdmin`` will provide the
+        correct model to use.
+
 .. method::  ModelAdmin.get_changelist_formset(self, request, **kwargs)
 
     Returns a :ref:`ModelFormSet <model-formsets>` class for use on the
@@ -1490,9 +1517,6 @@ needed. Now within your form you can add your own custom validation for
 any field::
 
     class MyArticleAdminForm(forms.ModelForm):
-        class Meta:
-            model = Article
-
         def clean_name(self):
             # do something that validates your data
             return self.cleaned_data["name"]