[1.9.x] Fixed #26526 -- Documented QuerySet.latest()'s dependency on database ordering.

Note that ``earliest()`` and ``latest()`` exist purely for convenience and

readability.

.. admonition:: ``earliest()`` and ``latest()`` may return instances with null dates.

    Since ordering is delegated to the database, results on fields that allow

    null values may be ordered differently if you use different databases. For

    example, PostgreSQL and MySQL sort null values as if they are higher than

    non-null values, while SQLite does the opposite.

    You may want to filter out null values::

        Entry.objects.filter(pub_date__isnull=False).latest('pub_date')

``earliest()``

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