gh-85133: os docs: Add that getenv uses os.environ (GH-91874)


Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit b25352a5c039d95e019dd8ca111f6f77c43ca1f7)

Co-authored-by: slateny <46876382+slateny@users.noreply.github.com>