Fixed #20224 -- Update docs examples which mention __unicode__

Thanks Marc Tamlyn and Tim Graham for the review.

docs/intro/overview.txt

@@ -31,6 +31,7 @@ the file ``mysite/news/models.py``::
     class Reporter(models.Model):
         full_name = models.CharField(max_length=70)
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return self.full_name
 
@@ -40,6 +41,7 @@ the file ``mysite/news/models.py``::
         content = models.TextField()
         reporter = models.ForeignKey(Reporter)
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return self.headline