Forms clean documentation omits essential

[Alper Cugun <djangoproject@…>]

Needed to dig into the source but the current documentation does not reflect that clean_FOO needs to return the cleaned value.

As in forms.py:
value = getattr(self, 'clean_%s' % name)()
self.cleaned_data[name] = value

So attached is a patch for the forms.txt documentation.

[James Bennett]

Duplicate of #7428.