Fixed #20997 -- Added SessionStore note to docs.

Using sessions out of views

===========================

.. note::

    The examples in this section import the ``SessionStore`` object directly

    from the ``django.contrib.sessions.backends.db`` backend. In your own code,

    you should consider importing ``SessionStore`` from the session engine

    designated by :setting:`SESSION_ENGINE`, as below:

      >>> from importlib import import_module

      >>> from django.conf import settings

      >>> SessionStore = import_module(settings.SESSION_ENGINE).SessionStore

An API is available to manipulate session data outside of a view::

    >>> from django.contrib.sessions.backends.db import SessionStore