Fixed #13020 - add clarifying note to SessionStore. thanks elbarto for the patch.

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

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

    >>> import datetime

    >>> s = SessionStore(session_key='2b1189a188b44ad18c35e113ac6ceead')

    >>> s['last_login'] = datetime.datetime(2005, 8, 20, 13, 35, 10)

    >>> s['last_login']

    datetime.datetime(2005, 8, 20, 13, 35, 0)

    >>> s.save()

If ``session_key`` isn't provided, one will be generated automatically::

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

    >>> s = SessionStore()

    >>> s.save()

    >>> s.session_key

    '2b1189a188b44ad18c35e113ac6ceead'

If you're using the ``django.contrib.sessions.backends.db`` backend, each

session is just a normal Django model. The ``Session`` model is defined in

``django/contrib/sessions/models.py``. Because it's a normal model, you can