Fixed #18315 -- Documented QueryDict.popitem and QueryDict.pop

        >>> q.lists()

        [(u'a', [u'1', u'2', u'3'])]

.. method:: QueryDict.pop(key)

    Returns a list of values for the given key and removes them from the

    dictionary. Raises ``KeyError`` if the key does not exist. For example::

        >>> q = QueryDict('a=1&a=2&a=3', mutable=True)

        >>> q.pop('a')

        [u'1', u'2', u'3']

.. method:: QueryDict.popitem()

    Removes an arbitrary member of the dictionary (since there's no concept

    of ordering), and returns a two value tuple containing the key and a list

    of all values for the key. Raises ``KeyError`` when called on an empty

    dictionary. For example::

        >>> q = QueryDict('a=1&a=2&a=3', mutable=True)

        >>> q.popitem()

        (u'a', [u'1', u'2', u'3'])

.. method:: QueryDict.dict()

    Returns ``dict`` representation of ``QueryDict``. For every (key, list)