Big glaring warning in docs/flatpages.txt to remind people how response middleware works

For more on middleware, read the `middleware docs`_.

.. admonition:: Ensure that your 404 template works

    Note that the ``FlatpageFallbackMiddleware`` only steps in once

    another view has successfully produced a 404 response. If another

    view or middleware class attempts to produce a 404 but ends up

    raising an exception instead (such as a ``TemplateDoesNotExist``

    exception if your site does not have an appropriate template to

    use for HTTP 404 responses), the response will become an HTTP 500

    ("Internal Server Error") and the ``FlatpageFallbackMiddleware``

    will not attempt to serve a flat page.

.. _SITE_ID: ../settings/#site-id

.. _RequestContext: ../templates_python/#subclassing-context-djangocontext

.. _middleware docs: ../middleware/