Fixed #7654: Documented the fact that file upload handlers must be modified...

.. note::

    You can only modify upload handlers *before* accessing ``request.FILES`` --

    it doesn't make sense to change upload handlers after upload handling has

    already started. If you try to modify ``request.upload_handlers`` after

    reading from ``request.FILES`` Django will throw an error.

    You can only modify upload handlers *before* accessing

    ``request.POST`` or ``request.FILES`` -- it doesn't make sense to

    change upload handlers after upload handling has already

    started. If you try to modify ``request.upload_handlers`` after

    reading from ``request.POST`` or ``request.FILES`` Django will

    throw an error.

    Thus, you should always modify uploading handlers as early in your view as

    possible.