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

R=golang-dev, bradfitz, r
CC=golang-dev
https://golang.org/cl/5700056
diff --git a/test/sigchld.go b/test/sigchld.go
index e7c3d5a..25625a6 100644
--- a/test/sigchld.go
+++ b/test/sigchld.go
@@ -5,6 +5,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Test that a program can survive SIGCHLD.
+
 package main
 
 import "syscall"