gh-100783: fix os.path.join documentation (GH-100811)

- Use "drive", not "drive letter", because of UNC paths
- Previous components are not thrown away from relative drive letters
- Use "segment" instead of "component" for consistency with pathlib
- Other miscellaneous improvements
(cherry picked from commit 53455a319f3f2e5609fca2a313ea356fba318665)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>