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
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go