Fixed #702 - Documented that ManyToMany fields can't be in unique_together; thanks poirier for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17314 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/ref/models/options.txt

@@ -247,6 +247,12 @@ Django quotes column and table names behind the scenes.
 
         unique_together = ("driver", "restaurant")
 
+    A :class:`~django.db.models.ManyToManyField` cannot be included in
+    unique_together (it's not even clear what that would mean). If you
+    need to validate uniqueness related to a
+    :class:`~django.db.models.ManyToManyField`, look at signals or
+    using an explicit :attr:`through <ManyToManyField.through>` model.
+
 ``verbose_name``
 ----------------