[3.13] gh-101100: Document zlib public constants to fix reference warnings (GH-139835)

(cherry picked from commit c7f1da9)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Author: 3 people