path/filepath: document that Walk does not follow symlinks
path/filepath: document that Walk does not follow symlinks

Fixes issue 4759.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7304043

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