[3.11] gh-87452: Improve the Popen.returncode docs (#103771)

Clarifies that it remains None until a method checking the child process
status has been called and noticed it has terminated.
(cherry picked from commit 68f583658247ceced323d79e1cf775c91c53c019)

Co-authored-by: Joshua Herman <30265+zitterbewegung@users.noreply.github.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>