Refs #10060 -- Corrected description of multiple annotations bug.

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Combining multiple aggregations with ``annotate()`` will `yield the wrong

results <https://code.djangoproject.com/ticket/10060>`_, as multiple tables are

cross joined. Due to the use of ``LEFT OUTER JOIN``, duplicate records will be

generated if some of the joined tables contain more records than the others:

results <https://code.djangoproject.com/ticket/10060>`_ because joins are used

instead of subqueries:

    >>> Book.objects.first().authors.count()

    2

rating exceeding 3.0, hence publisher C is excluded.

In the first query, the annotation precedes the filter, so the filter has no

effect on the annotation. ``distinct=True`` is required to avoid a

:ref:`cross-join bug <combining-multiple-aggregations>`.

effect on the annotation. ``distinct=True`` is required to avoid a :ref:`query

bug <combining-multiple-aggregations>`.

The second query counts the number of books that have a rating exceeding 3.0

for each publisher. The filter precedes the annotation, so the filter