Fixed #20183 - Clarified docs assumption that certain objects exist in database.

Updating a :class:`~django.db.models.ForeignKey` field works exactly the same

way as saving a normal field -- simply assign an object of the right type to

the field in question. This example updates the ``blog`` attribute of an

``Entry`` instance ``entry``::

``Entry`` instance ``entry``, assuming appropriate instances of ``Entry`` and

``Blog`` are already saved to the database (so we can retrieve them below)::

    >>> from blog.models import Entry

    >>> entry = Entry.objects.get(pk=1)