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
https://golang.org/cl/4977059

src/pkg/path/filepath/path.go

@@ -259,8 +259,7 @@ func Abs(path string) (string, os.Error) {
 }
 
 // Visitor methods are invoked for corresponding file tree entries
-// visited by Walk. The parameter path is the full path of f relative
-// to root.
+// visited by Walk. The provided path parameter begins with root.
 type Visitor interface {
 	VisitDir(path string, f *os.FileInfo) bool
 	VisitFile(path string, f *os.FileInfo)