unicode: Added a comment to the example in [5602].

git-svn-id: http://code.djangoproject.com/svn/django/branches/unicode@5603 bcc190cf-cafb-0310-a4f2-bffc1f526a37

tests/modeltests/str/models.py

@@ -22,6 +22,8 @@ class Article(models.Model):
     pub_date = models.DateTimeField()
 
     def __str__(self):
+        # Caution: this is only safe if you are certain that headline will be
+        # in ASCII.
         return self.headline
 
 class InternationalArticle(models.Model):