[doc] Clarify exception in `multiprocessing.cpu_count` (GH-23660) (GH-28433)

Previous wording didn't explain the slightly unintuitive behavior.

Co-authored-by: kj <28750310+Fidget-Spinner@users.noreply.github.com>
(cherry picked from commit 24dbe30f8df80740704db3743d071b3218d1276e)

Co-authored-by: Emmanuel Arias <eamanu@yaerobi.com>