test: commentary for [h-m]*.go

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5674112
diff --git a/test/label.go b/test/label.go
index 8f2df4c..b30c27e 100644
--- a/test/label.go
+++ b/test/label.go
@@ -4,7 +4,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Pass 1 label errors.
+// Verify that erroneous labels are caught by the compiler.
+// This set is caught by pass 1.
+// Does not compile.
 
 package main