Fixed #22268 -- Documented values_list() behavior for multivalued relations.a

    >>> Entry.objects.values_list('headline', flat=True).get(pk=1)

    'First entry'

``values()`` and ``values_list()`` are both intended as optimizations for a

specific use case: retrieving a subset of data without the overhead of creating

a model instance. This metaphor falls apart when dealing with many-to-many and

other multivalued relations (such as the one-to-many relation of a reverse

foreign key) because the the "one row, one object" assumption doesn't hold.

For example, notice the behavior when querying across a

:class:`~django.db.models.ManyToManyField`::

    >>> Author.objects.values_list('name', 'entry__headline')

    [('Noam Chomsky', 'Impressions of Gaza'),

     ('George Orwell', 'Why Socialists Do Not Believe in Fun'),

     ('George Orwell', 'In Defence of English Cooking'),

     ('Don Quixote', None)]

Authors with multiple entries appear multiple times and authors without any

entries have ``None`` for the entry headline.

Similarly, when querying a reverse foreign key, ``None`` appears for entries

not having any author::

    >>> Entry.objects.values_list('authors')

    [('Noam Chomsky',), ('George Orwell',), (None,)]

``dates()``

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