path/filepath: fix Visitor doc
path/filepath: fix Visitor doc

The path is not in fact relative to the root, but
joined to it.

R=golang-dev, adg, rsc, gustavo
CC=golang-dev
http://codereview.appspot.com/4977059

wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old