Fixed #20224 -- Update docs examples which mention __unicode__

Thanks Marc Tamlyn and Tim Graham for the review.

docs/topics/class-based-views/generic-display.txt

@@ -89,6 +89,7 @@ We'll be using these models::
         class Meta:
             ordering = ["-name"]
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return self.name
 
@@ -98,6 +99,7 @@ We'll be using these models::
         email = models.EmailField()
         headshot = models.ImageField(upload_to='author_headshots')
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return self.name