[3.14] gh-94632: document the subprocess need for extra_groups=() with user= (GH-148129) (#148130)

gh-94632: document the subprocess need for extra_groups=() with user= (GH-148129)
(cherry picked from commit a1cf4430ed89ec702528ef074138c407ccf89946)

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>