Added some doc links for django.contrib.messages

The messages framework can use different backends to store temporary messages.

Django provides three built-in storage classes:

Django provides three built-in storage classes in

:mod:`django.contrib.messages`:

.. class:: django.contrib.messages.storage.session.SessionStorage

.. class:: storage.session.SessionStorage

    This class stores all messages inside of the request's session. Therefore

    it requires Django's ``contrib.sessions`` application.

.. class:: django.contrib.messages.storage.cookie.CookieStorage

.. class:: storage.cookie.CookieStorage

    This class stores the message data in a cookie (signed with a secret hash

    to prevent manipulation) to persist notifications across requests. Old

    messages are dropped if the cookie data size would exceed 2048 bytes.

.. class:: django.contrib.messages.storage.fallback.FallbackStorage

.. class:: storage.fallback.FallbackStorage

    This class first uses ``CookieStorage``, and falls back to using

    ``SessionStorage`` for the messages that could not fit in a single cookie.

    MESSAGE_STORAGE = 'django.contrib.messages.storage.cookie.CookieStorage'

.. class:: storage.base.BaseStorage

To write your own storage class, subclass the ``BaseStorage`` class in

``django.contrib.messages.storage.base`` and implement the ``_get`` and

``_store`` methods.

See :ref:`message storage backends <message-storage-backends>` for more details.

The backends that use cookies -- ``CookieStorage`` and ``FallbackStorage`` --

The backends that use cookies --

:class:`~django.contrib.messages.storage.cookie.CookieStorage` and

:class:`~django.contrib.messages.storage.fallback.FallbackStorage` --

use the value of :setting:`SESSION_COOKIE_DOMAIN` when setting their cookies.

.. setting:: MESSAGE_TAGS