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

Thanks Sai Krishna for the initial patch.

docs/ref/models/querysets.txt

@@ -637,6 +637,30 @@ achieve that, use ``values_list()`` followed by a ``get()`` call::
     >>> 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()``
 ~~~~~~~~~~~