[3.13] gh-112346: Document the OS byte in `gzip.compress` output change in 3.11 (GH-120480) (#120612)

gh-112346: Document the OS byte in `gzip.compress` output change in 3.11 (GH-120480)
(cherry picked from commit bac4edad69bb20dd9460766e062637cae999e1e0)


gh-112346: Describe the "os" byte in gzip output change.

Co-authored-by: Gregory P. Smith <greg@krypto.org>