Fixed #20224 -- Update docs examples which mention __unicode__

Thanks Marc Tamlyn and Tim Graham for the review.

docs/internals/contributing/writing-code/coding-style.txt

@@ -124,13 +124,16 @@ Model style
           first_name = models.CharField(max_length=20)
           last_name = models.CharField(max_length=40)
 
+* If you define a ``__str__`` method (previously ``__unicode__`` before Python 3
+  was supported), decorate the model class with
+  :func:`~django.utils.encoding.python_2_unicode_compatible`.
+
 * The order of model inner classes and standard methods should be as
   follows (noting that these are not all required):
 
   * All database fields
   * Custom manager attributes
   * ``class Meta``
-  * ``def __unicode__()``
   * ``def __str__()``
   * ``def save()``
   * ``def get_absolute_url()``