win, doc: document per-drive current working dir

Add note to fs.md and path.md about Windows using per-drive current
working directory.

Fixes: https://github.com/nodejs/node/issues/9378
PR-URL: https://github.com/nodejs/node/pull/13330


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>