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