[3.14] gh-138234: clarify returncode behavior for subprocesses created with `shell=True` (GH-138536) (#146254)

gh-138234: clarify returncode behavior for subprocesses created with `shell=True` (GH-138536)
(cherry picked from commit 8a531f89df8f8bf4c4fe395f9edcdc19852bdf1c)

Co-authored-by: andreuu-tsai <32549555+andreuu-tsai@users.noreply.github.com>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>