[1.6.x] Fixed #20859 - Clarified Model.clean() example.

Backport of 94d7fed775 from master

AUTHORS

@@ -540,6 +540,7 @@ answer newbie questions, and generally made Django that much better:
     smurf@smurf.noris.de
     Vsevolod Solovyov
     George Song <george@damacy.net>
+    Jimmy Song <jaejoon@gmail.com>
     sopel
     Leo Soto <leo.soto@gmail.com>
     Wiliam Alves de Souza <wiliamsouza83@gmail.com>

docs/ref/models/instances.txt

@@ -140,9 +140,13 @@ attributes on your model if desired. For instance, you could use it to
 automatically provide a value for a field, or to do validation that requires
 access to more than a single field::
 
-    def clean(self):
     import datetime
     from django.core.exceptions import ValidationError
+    from django.db import models
+
+    class Article(models.Model):
+        ...
+        def clean(self):
             # Don't allow draft entries to have a pub_date.
             if self.status == 'draft' and self.pub_date is not None:
                 raise ValidationError('Draft entries may not have a publication date.')