Fixed #21894 -- Corrected a form.clean() example in case a superclass doesn't return data.

In this code, if the validation error is raised, the form will display an

error message at the top of the form (normally) describing the problem.

Note that the call to ``super(ContactForm, self).clean()`` in the example code

ensures that any validation logic in parent classes is maintained.

The second approach might involve assigning the error message to one of the

fields. In this case, let's assign an error message to both the "subject" and

"cc_myself" rows in the form display. Be careful when doing this in practice,

since it can lead to confusing form output. We're showing what is possible

here and leaving it up to you and your designers to work out what works

effectively in your particular situation. Our new code (replacing the previous

sample) looks like this::

The call to ``super(ContactForm, self).clean()`` in the example code ensures

that any validation logic in parent classes is maintained. If your form

inherits another that doesn't return a ``cleaned_data`` dictionary in its

``clean()`` method (doing so is optional), then don't assign ``cleaned_data``

to the result of the ``super()`` call and use ``self.cleaned_data`` instead::

    def clean(self):

        super(ContactForm, self).clean()

        cc_myself = self.cleaned_data.get("cc_myself")

        ...

The second approach for reporting validation errors might involve assigning the

error message to one of the fields. In this case, let's assign an error message

to both the "subject" and "cc_myself" rows in the form display. Be careful when

doing this in practice, since it can lead to confusing form output. We're

showing what is possible here and leaving it up to you and your designers to

work out what works effectively in your particular situation. Our new code

(replacing the previous sample) looks like this::

    from django import forms