[3.12] gh-90300: Fix cmdline.rst (GH-116721) (GH-116724)

* Fix the description of the "-b" option.
* Add references to environment variables for "-s" and "-X dev" options.
(cherry picked from commit 33662d4e01d73cd4f29a25efc2ef09288129023f)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>