[3.12] gh-108224: Fix asyncio doc inconsistency (GH-108230) (#108231)

gh-108224: Fix asyncio doc inconsistency (GH-108230)

(Spawning subprocesses does not require the event loop to run in the main thread -- only signal handling does.)
(cherry picked from commit 1cc391d9e2ea24ca750005335507b52933fc0b52)

Co-authored-by: temach <tematibr@gmail.com>