Document that embedded models don't have a primary key value

Author: timgraham

docs/source/ref/models/models.rst

@@ -13,3 +13,6 @@ must subclass.
 Since these models are not stored in their own collection, they do not have
 any of the normal ``QuerySet`` methods (``all()``, ``filter()``, ``delete()``,
 etc.) You also cannot call ``Model.save()`` and ``delete()`` on them.
+
+Embedded model instances won't have a value for their primary key unless one is
+explicitly set.