Fixed #20444 -- Cookie-based sessions does not include a remote code execution-warning

.. warning::

    **If the :setting:`SECRET_KEY` is not kept secret, this can lead to

    arbitrary remote code execution.**

    An attacker in possession of the :setting:`SECRET_KEY` can not only

    generate falsified session data, which your site will trust, but also

    remotely execute arbitrary code, as the data is serialized using pickle.

    If you use cookie-based sessions, pay extra care that your secret key is

    always kept completely secret, for any system which might be remotely

    accessible.

    **The session data is signed but not encrypted**

    When using the cookies backend the session data can be read by the client.