[3.14] gh-136437: Document some `os.path` functions as requiring pos-only (GH-136812) (#136944)

gh-136437: Document some `os.path` functions as requiring pos-only (GH-136812)
(cherry picked from commit b5428bb0e786f5b67c6077472c0068cadd0b5ea9)

Co-authored-by: sobolevn <mail@sobolevn.me>