Clarified timeout=0 in cache docs.

behavior. These arguments are provided as additional keys in the

:setting:`CACHES` setting. Valid arguments are as follows:

* :setting:`TIMEOUT <CACHES-TIMEOUT>`: The default timeout, in

  seconds, to use for the cache. This argument defaults to ``300`` seconds (5 minutes).

  You can set ``TIMEOUT`` to ``None`` so that, by default, cache keys never

  expire.

  expire. A value of ``0`` causes keys to immediately expire (effectively

  "don't cache").

* :setting:`OPTIONS <CACHES-OPTIONS>`: Any options that should be

  passed to the cache backend. The list of valid options will vary

The ``timeout`` argument is optional and defaults to the ``timeout`` argument

of the appropriate backend in the :setting:`CACHES` setting (explained above).

It's the number of seconds the value should be stored in the cache. Passing in

``None`` for ``timeout`` will cache the value forever.

``None`` for ``timeout`` will cache the value forever. A ``timeout`` of ``0``

won't cache the value.

If the object doesn't exist in the cache, ``cache.get()`` returns ``None``::