Changed note about update_fields and pre_save() to admonition.

Follow up to bf47c719719d0e190a99fa2e7f959d5bbb7caf8a.

docs/ref/models/instances.txt

@@ -621,7 +621,7 @@ from the DB will get updated. In effect there is an automatic
 ``update_fields`` in this case. If you assign or change any deferred field
 value, the field will be added to the updated fields.
 
-.. note::
+.. admonition:: ``Field.pre_save()`` and ``update_fields``
 
     If ``update_fields`` is passed in, only the
     :meth:`~django.db.models.Field.pre_save` methods of the ``update_fields``