doc: use simpler language for NULL return from ANY/ALL
Previously the combination of "does not return" and "any row" caused
ambiguity.
Reported-by: KES <kes-kes@yandex.ru>
Discussion: https://postgr.es/m/
153701242703.22334.
1476830122267077397@wrigleys.postgresql.org
Reviewed-by: David G. Johnston
Backpatch-through: 9.3