Improve docs about using ORDER BY to control aggregate input order.

Improve docs about using ORDER BY to control aggregate input order.

David Johnston pointed out that the original text here had been obsoleted
by SQL:2008, which allowed ORDER BY in subqueries.  We could weaken the
text to describe ORDER-BY-in-subqueries as an optional SQL feature that's
possibly unportable; but then the exact same statements would apply to
the alternative it's being compared to (ORDER-BY-in-aggregate-calls).
So really that would be pretty useless; let's just take out the sentence
entirely.  Instead, point out the hazard that any extra processing in the
upper query might cause the subquery output order to be destroyed.

Discussion: <CAKFQuwbAX=iO9QbpN7_jr+BnUWm9FYX8WbEPUvG0p+nZhp6TZg@mail.gmail.com>