[3.12] gh-114539: Clarify implicit launching of shells by subprocess (GH-117996) (#118002)

gh-114539: Clarify implicit launching of shells by subprocess (GH-117996)
(cherry picked from commit a4b44d39cd6941cc03590fee7538776728bdfd0a)

Co-authored-by: Steve Dower <steve.dower@python.org>