describe each interface test

R=r
DELTA=20  (20 added, 0 deleted, 0 changed)
OCL=28707
CL=28716
diff --git a/test/interface6.go b/test/interface6.go
index c675595..49926f0 100644
--- a/test/interface6.go
+++ b/test/interface6.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.
 
+// Interface values containing structures.
+
 package main
 
 import "os"