Fixed #20224 -- Update docs examples which mention __unicode__

Thanks Marc Tamlyn and Tim Graham for the review.

docs/ref/models/instances.txt

@@ -430,6 +430,12 @@ Other model instance methods
 
 A few object methods have special purposes.
 
+.. note::
+    On Python 3, as all strings are natively considered Unicode, only use the
+    ``__str__()`` method (the ``__unicode__()`` method is obsolete).
+    If you'd like compatibility with Python 2, you can decorate your model class
+    with :func:`~django.utils.encoding.python_2_unicode_compatible`.
+
 ``__unicode__``
 ---------------
 

docs/ref/models/querysets.txt

@@ -814,6 +814,7 @@ For example, suppose you have these models::
         name = models.CharField(max_length=50)
         toppings = models.ManyToManyField(Topping)
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return u"%s (%s)" % (self.name, u", ".join([topping.name
                                                         for topping in self.toppings.all()]))