Clarified documentation regarding string-form model references.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3597 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/model-api.txt

@@ -686,8 +686,9 @@ you can use the name of the model, rather than the model object itself::
     class Manufacturer(models.Model):
         # ...
 
-Note, however, that support for strings around model names in ``ForeignKey`` is
-quite new, and it can be buggy in some cases.
+Note, however, that you can only use strings to refer to models in the same 
+models.py file - you cannot use a string to reference a model in a different 
+application, or to reference a model that has been imported from elsewhere.
 
 Behind the scenes, Django appends ``"_id"`` to the field name to create its
 database column name. In the above example, the database table for the ``Car``
@@ -809,7 +810,10 @@ here's how you'd represent that::
 
 As with ``ForeignKey``, a relationship to self can be defined by using the
 string ``'self'`` instead of the model name, and you can refer to as-yet
-undefined models by using a string containing the model name.
+undefined models by using a string containing the model name. However, you 
+can only use strings to refer to models in the same models.py file - you 
+cannot use a string to reference a model in a different application, or to 
+reference a model that has been imported from elsewhere.
 
 It's suggested, but not required, that the name of a ``ManyToManyField``
 (``toppings`` in the example above) be a plural describing the set of related