Update customizing.txt

The origin statement "which could be ... or whatever" **misguides** many newbies like me.

In fact, the ``login`` function in ``contrib.auth`` stores ``user.pk`` in session, then ``get_user`` function in ``contrib.auth`` gets ``user.pk`` in session and then passes it to your custom ``get_user`` as ``user_id``.

Which means, ``user_id`` prarameter in your custom ``get_user`` has to be the primary key of ``User`` object, too.

docs/topics/auth/customizing.txt

@@ -95,7 +95,8 @@ An authentication backend is a class that implements two required methods:
 optional permission related :ref:`authorization methods <authorization_methods>`.
 
 The ``get_user`` method takes a ``user_id`` -- which could be a username,
-database ID or whatever -- and returns a ``User`` object.
+database ID or whatever, but has to be the primary key of your ``User`` object
+-- and returns a ``User`` object.
 
 The ``authenticate`` method takes credentials as keyword arguments. Most of
 the time, it'll just look like this::