test: explanatory comments [c-g]*

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5656103
diff --git a/test/func3.go b/test/func3.go
index 587b7c4..6be3bf0 100644
--- a/test/func3.go
+++ b/test/func3.go
@@ -4,6 +4,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Verify that illegal function signatures are detected.
+// Does not compile.
+
 package main
 
 type t1 int