[1.6.x] Fixed #22473 -- Emphasized that Model.clean() is not called from Model.save().

            if self.status == 'published' and self.pub_date is None:

                self.pub_date = datetime.date.today()

Note, however, that like :meth:`Model.full_clean()`, a model's ``clean()``

method is not invoked when you call your model's :meth:`~Model.save()` method.

Any :exc:`~django.core.exceptions.ValidationError` exceptions raised by

``Model.clean()`` will be stored in a special key error dictionary key,

``NON_FIELD_ERRORS``, that is used for errors that are tied to the entire model