Fixed #20124 - Fixed doc warnings.

.. versionadded:: 1.6

.. class:: django.contrib.messages.views.SuccessMessageMixin

.. class:: views.SuccessMessageMixin

    Adds a success message attribute to

    :class:`~django.views.generic.edit.FormView` based classes

* Added :class:`~django.contrib.messages.views.SuccessMessageMixin` which

  provides a ``success_message`` attribute for

  :class:`~django.view.generic.edit.FormView` based classes.

  :class:`~django.views.generic.edit.FormView` based classes.

* Added the :attr:`django.db.models.ForeignKey.db_constraint` and

  :attr:`django.db.models.ManyToManyField.db_constraint` options.

* The :djadmin:`diffsettings` comand gained a ``--all`` option.

* :func:`django.forms.fields.Field.__init__` now calls ``super()``, allowing

* ``django.forms.fields.Field.__init__`` now calls ``super()``, allowing

  field mixins to implement ``__init__()`` methods that will reliably be

  called.