Fixed #20224 -- Update docs examples which mention __unicode__

Thanks Marc Tamlyn and Tim Graham for the review.

docs/topics/db/examples/many_to_many.txt

@@ -16,6 +16,7 @@ objects, and a ``Publication`` has multiple ``Article`` objects:
     class Publication(models.Model):
         title = models.CharField(max_length=30)
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return self.title
 
@@ -26,6 +27,7 @@ objects, and a ``Publication`` has multiple ``Article`` objects:
         headline = models.CharField(max_length=100)
         publications = models.ManyToManyField(Publication)
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return self.headline
 

docs/topics/db/examples/many_to_one.txt

@@ -15,6 +15,7 @@ To define a many-to-one relationship, use :class:`~django.db.models.ForeignKey`.
         last_name = models.CharField(max_length=30)
         email = models.EmailField()
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return u"%s %s" % (self.first_name, self.last_name)
 
@@ -23,6 +24,7 @@ To define a many-to-one relationship, use :class:`~django.db.models.ForeignKey`.
         pub_date = models.DateField()
         reporter = models.ForeignKey(Reporter)
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return self.headline
 
@@ -56,9 +58,9 @@ Article objects have access to their related Reporter objects::
 
     >>> r = a.reporter
 
-These are strings instead of unicode strings because that's what was used in
-the creation of this reporter (and we haven't refreshed the data from the
-database, which always returns unicode strings)::
+On Python 2, these are strings of type ``str`` instead of unicode strings
+because that's what was used in the creation of this reporter (and we haven't
+refreshed the data from the database, which always returns unicode strings)::
 
     >>> r.first_name, r.last_name
     ('John', 'Smith')

docs/topics/db/examples/one_to_one.txt

@@ -16,6 +16,7 @@ In this example, a ``Place`` optionally can be a ``Restaurant``:
         name = models.CharField(max_length=50)
         address = models.CharField(max_length=80)
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return u"%s the place" % self.name
 
@@ -24,6 +25,7 @@ In this example, a ``Place`` optionally can be a ``Restaurant``:
         serves_hot_dogs = models.BooleanField()
         serves_pizza = models.BooleanField()
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return u"%s the restaurant" % self.place.name
 
@@ -31,6 +33,7 @@ In this example, a ``Place`` optionally can be a ``Restaurant``:
         restaurant = models.ForeignKey(Restaurant)
         name = models.CharField(max_length=50)
 
+        # On Python 3: def __str__(self):
         def __unicode__(self):
             return u"%s the waiter at %s" % (self.name, self.restaurant)