path/filepath: document that Walk sorts its output

R=golang-dev, cw
CC=golang-dev
https://golang.org/cl/5004045

src/pkg/path/filepath/path.go

@@ -302,7 +302,9 @@ func walk(path string, info *os.FileInfo, walkFn WalkFunc) os.Error {
 
 // Walk walks the file tree rooted at root, calling walkFn for each file or
 // directory in the tree, including root. All errors that arise visiting files
-// and directories are filtered by walkFn.
+// and directories are filtered by walkFn. The files are walked in lexical
+// order, which makes the output deterministic but means that for very
+// large directories Walk can be inefficient.
 func Walk(root string, walkFn WalkFunc) os.Error {
 	info, err := os.Lstat(root)
 	if err != nil {