Fixed #21422 -- Documented that prefetch_related() results must be homogeneous.

``select_related``, in addition to the foreign key and one-to-one relationships

that are supported by ``select_related``. It also supports prefetching of

:class:`~django.contrib.contenttypes.fields.GenericRelation` and

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

:class:`~django.contrib.contenttypes.fields.GenericForeignKey`, however, it

must be restricted to a homogeneous set of results. For example, prefetching

objects referenced by a ``GenericForeignKey`` is only supported if the query

is restricted to one ``ContentType``.

For example, suppose you have these models::