Remove outdated ForeignKey manager documentation.

    >>> b.entries.filter(headline__contains='Lennon')

    >>> b.entries.count()

You cannot access a reverse :class:`~django.db.models.ForeignKey`

:class:`~django.db.models.Manager` from the class; it must be accessed from an

instance::

    >>> Blog.entry_set

    Traceback:

        ...

    AttributeError: "Manager must be accessed via instance".

In addition to the :class:`~django.db.models.query.QuerySet` methods defined in

"Retrieving objects" above, the :class:`~django.db.models.ForeignKey`

:class:`~django.db.models.Manager` has additional methods used to handle the