test: document ken/*.go

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5694065

test/ken/cplx2.go

diff --git a/test/ken/cplx2.go b/test/ken/cplx2.go
index a3c1570..eb1da7b 100644
--- a/test/ken/cplx2.go
+++ b/test/ken/cplx2.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 arithmetic on complex numbers, including multiplication and division.
+
 package main
 
 const (