Fixed #17026 -- Improved wording of contrib.messages' storage backends section

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

To change which backend is being used, add a `MESSAGE_STORAGE`_ to your

If the default FallbackStorage isn't suitable to your needs, you can change

which backend is being used by adding a `MESSAGE_STORAGE`_ to your

settings, referencing the module and class of the storage class. For

example::

The value should be the full path of the desired storage class.

Four storage classes are included:

Three storage classes are available:

``'django.contrib.messages.storage.session.SessionStorage'``

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

    messages are dropped if the cookie data size would exceed 4096 bytes.

``'django.contrib.messages.storage.fallback.FallbackStorage'``

    This is the default storage class.

    This class first uses CookieStorage for all messages, falling back to using

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