test/[n-z]*.go: add documentation

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/5700056
diff --git a/test/stringrange.go b/test/stringrange.go
index a1534c4..daaba91 100644
--- a/test/stringrange.go
+++ b/test/stringrange.go
@@ -4,6 +4,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Test range over strings.
+
 package main
 
 import (