[1.7.x] Fixed #22786 -- Documented value_from_datadict caveat

    .. method:: value_from_datadict(data, files, name)

        Given a dictionary of data and this widget's name, returns the value

        of this widget. Returns ``None`` if a value wasn't provided.

        of this widget. ``files`` may contain data coming from

        :attr:`request.FILES <django.http.HttpRequest.FILES>`. Returns ``None``

        if a value wasn't provided. Note also that ``value_from_datadict`` may

        be called more than once during handling of form data, so if you

        customize it and add expensive processing, you should implement some

        caching mechanism yourself.

.. class:: MultiWidget(widgets, attrs=None)