Fixed #9255: Removed now-obsolete warning about the old semantics of __exact=None vs. __isnull=True

    SELECT ... WHERE pub_date IS NULL;

.. admonition:: ``__isnull=True`` vs ``__exact=None``

    There is an important difference between ``__isnull=True`` and

    ``__exact=None``. ``__exact=None`` will *always* return an empty result

    set, because SQL requires that no value is equal to ``NULL``.

    ``__isnull`` determines if the field is currently holding the value

    of ``NULL`` without performing a comparison.

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