[3.13] gh-106318: Improve str.rstrip() method doc (GH-143893) (#144698)

gh-106318: Improve str.rstrip() method doc (GH-143893)
(cherry picked from commit 936d60dbe1679f05d7ceb0a6d1f65bc741390ac6)

Co-authored-by: Adorilson Bezerra <adorilson@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>