Fixed #21951 -- Updated docs to use __str__ for Python 3

Thanks Tim Graham for the report and recommendations

docs/ref/models/instances.txt

@@ -463,9 +463,29 @@ the conversion to string objects when required.
 
 .. method:: Model.__str__()
 
-The ``__str__()`` method is called whenever you call ``str()`` on an object. The main use for this method directly inside Django is when the ``repr()`` output of a model is displayed anywhere (for example, in debugging output).
-Thus, you should return a nice, human-readable string for the object's
-``__str__()``.  It isn't required to put ``__str__()`` methods everywhere if you have sensible :meth:`~Model.__unicode__()` methods.
+The ``__str__()`` method is called whenever you call ``str()`` on an
+object. In Python 3, Django uses ``str(obj)`` in a number of
+places. Most notably, to display an object in the Django admin site
+and as the value inserted into a template when it displays an
+object. Thus, you should always return a nice, human-readable
+representation of the model from the ``__str__()`` method.
+
+For example::
+
+    from django.db import models
+
+    class Person(models.Model):
+        first_name = models.CharField(max_length=50)
+        last_name = models.CharField(max_length=50)
+
+        def __str__(self):
+            return '%s %s' % (self.first_name, self.last_name)
+
+In Python 2, the main use of ``__str__`` directly inside Django is
+when the ``repr()`` output of a model is displayed anywhere (for
+example, in debugging output). It isn't required to put ``__str__()``
+methods everywhere if you have sensible :meth:`~Model.__unicode__()`
+methods.
 
 The previous :meth:`~Model.__unicode__()` example could be similarly written
 using ``__str__()`` like this::

docs/ref/models/querysets.txt

@@ -812,17 +812,16 @@ 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()]))
+        def __str__(self):              # __unicode__ on Python 2
+            return "%s (%s)" % (self.name, ", ".join([topping.name
+                                                      for topping in self.toppings.all()]))
 
 and run::
 
     >>> Pizza.objects.all()
-    [u"Hawaiian (ham, pineapple)", u"Seafood (prawns, smoked salmon)"...
+    ["Hawaiian (ham, pineapple)", "Seafood (prawns, smoked salmon)"...
 
-The problem with this is that every time ``Pizza.__unicode__()`` asks for
+The problem with this is that every time ``Pizza.__str__()`` asks for
 ``self.toppings.all()`` it has to query the database, so
 ``Pizza.objects.all()`` will run a query on the Toppings table for **every**
 item in the Pizza ``QuerySet``.