Fixed #20243 - Clarified when RelatedManager.remove() exists.

            >>> e = Entry.objects.get(id=234)

            >>> b.entry_set.remove(e) # Disassociates Entry e from Blog b.

        In order to prevent database inconsistency, this method only exists on

        :class:`~django.db.models.ForeignKey` objects where ``null=True``. If

        the related field can't be set to ``None`` (``NULL``), then an object

        can't be removed from a relation without being added to another. In the

        above example, removing ``e`` from ``b.entry_set()`` is equivalent to

        doing ``e.blog = None``, and because the ``blog``

        :class:`~django.db.models.ForeignKey` doesn't have ``null=True``, this

        is invalid.

        For :class:`~django.db.models.ForeignKey` objects, this method only

        exists if ``null=True``. If the related field can't be set to ``None``

        (``NULL``), then an object can't be removed from a relation without

        being added to another. In the above example, removing ``e`` from

        ``b.entry_set()`` is equivalent to doing ``e.blog = None``, and because

        the ``blog`` :class:`~django.db.models.ForeignKey` doesn't have

        ``null=True``, this is invalid.

    .. method:: clear()