test/[n-r]*.go: add documentation
The rename ones needed redoing.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5698054
diff --git a/test/nil.go b/test/nil.go
index fd93827..9f7bcbb 100644
--- a/test/nil.go
+++ b/test/nil.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 nil.
+
 package main
 
 import (