Update 3.0-announcement.md

The recommended workaround for simulating the old "full_clean" method wouldn't work for update(), as the full set of attrs to build out the model instance would not be provided.

docs/topics/3.0-announcement.md

@@ -237,7 +237,12 @@ The one difference that you do need to note is that the `.clean()` method will n
 There may be some cases where you really do need to keep validation logic in the model `.clean()` method, and cannot instead separate it into the serializer `.validate()`. You can do so by explicitly instantiating a model instance in the `.validate()` method.
 
     def validate(self, attrs):
-        instance = ExampleModel(**attrs)
+        if not self.instance:
+            instance = self.Meta.model(**attrs)
+        else:
+            instance = self.instance
+            for attr_name in attrs:
+                instance.attr_name = attrs.get('attr_name')
         instance.clean()
         return attrs