Small generic FK docs tweaks.

to" one other model, which means that if the ``TaggedItem`` model used a

:class:`~django.db.models.ForeignKey` it would have to

choose one and only one model to store tags for. The contenttypes

application provides a special field type which

application provides a special field type (``GenericForeignKey``) which

works around this and allows the relationship to be with any

model:

    :class:`~django.contrib.contenttypes.generic.GenericForeignKey`:

    1. Give your model a :class:`~django.db.models.ForeignKey`

       to :class:`~django.contrib.contenttypes.models.ContentType`.

       to :class:`~django.contrib.contenttypes.models.ContentType`. The usual

       name for this field is "content_type".

    2. Give your model a field that can store primary key values from the

       models you'll be relating to. For most models, this means a