Added support for negative indexes to PurePath.parents (GH-21799)

This commit also fixes up some of the overlapping documentation changed
in bpo-35498, which added support for indexing with slices.

Fixes bpo-21041.
https://bugs.python.org/issue21041

Co-authored-by: Paul Ganssle <p.ganssle@gmail.com>
Co-authored-by: Rémi Lapeyre <remi.lapeyre@henki.fr>