path/filepath: remove named result parameter for VolumeName

Fix style by removing unnecessary named result parameter.

Fix doc comment while here.

Change-Id: If8394e696ab37e00a95484d5137955aa06c59520
Reviewed-on: https://go-review.googlesource.com/1781
Reviewed-by: Yasuhiro MATSUMOTO <mattn.jp@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>