Add a Hint to __unicode__ exercise for reader

[riseriyo]

Suggestion: Add a Hint for the reader:

e.g.,
"When writing your unicode method for your Comment model, make sure you know what you are returning as a value. If you are not sure, take a look at our test, test_unicode_representation, again and see what it is asserting."

[treyhunner]

@riseriyo where should note be added? In the __str__ test in part 2 or in part 3

[riseriyo]

@treyhunner In part 3, we ask the reader to do an exercise. And yes, it seems we are now using __str__ instead of __unicode__ so it's referring to the test, test_string_representation in the CommentModelTest. I'm not sure if this hint is needed anymore. Thoughts?