Fixed #24414 -- Added examples of Prefetch object usage to the docs.

The ``lookup`` argument describes the relations to follow and works the same

as the string based lookups passed to

:meth:`~django.db.models.query.QuerySet.prefetch_related()`.

:meth:`~django.db.models.query.QuerySet.prefetch_related()`. For example:

    >>> Question.objects.prefetch_related(Prefetch('choice_set')).get().choice_set.all()

    [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]

    # This will only execute two queries regardless of the number of Question

    # and Choice objects.

    >>> Question.objects.prefetch_related(Prefetch('choice_set')).all()

    [<Question: Question object>]

The ``queryset`` argument supplies a base ``QuerySet`` for the given lookup.

This is useful to further filter down the prefetch operation, or to call

:meth:`~django.db.models.query.QuerySet.select_related()` from the prefetched

relation, hence reducing the number of queries even further.

relation, hence reducing the number of queries even further:

    >>> voted_choices = Choice.objects.filter(votes__gt=0)

    >>> voted_choices

    [<Choice: The sky>]

    >>> prefetch = Prefetch('choice_set', queryset=voted_choices)

    >>> Question.objects.prefetch_related(prefetch).get().choice_set.all()

    [<Choice: The sky>]

The ``to_attr`` argument sets the result of the prefetch operation to a custom

attribute.

attribute:

    >>> prefetch = Prefetch('choice_set', queryset=voted_choices, to_attr='voted_choices')

    >>> Question.objects.prefetch_related(prefetch).get().voted_choices

    [<Choice: The sky>]

    >>> Question.objects.prefetch_related(prefetch).get().choice_set.all()

    [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]

.. note::