path, path/filepath: add Join example with joined rooted path

path, path/filepath: add Join example with joined rooted path

This makes clear that Go's path.Join and filepath.Join are different
from the Python os.path.join (and perhaps others).

Requested in private mail.

Change-Id: Ie5dfad8a57f9baa5cca31246af1fd4dd5b1a64ee
Reviewed-on: https://go-review.googlesource.com/20711
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>