reflect: explicitly state that Type values can be used as map keys

Fixes #6535

Change-Id: I34974c0050424c96d19ad69bf4522bb69cde2fd5
Reviewed-on: https://go-review.googlesource.com/85815Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/reflect/type.go

@@ -32,7 +32,8 @@ import (
 // calling kind-specific methods. Calling a method
 // inappropriate to the kind of type causes a run-time panic.
 //
-// Type values are comparable, such as with the == operator.
+// Type values are comparable, such as with the == operator, and Type can be
+// used as a map key type.
 // Two Type values are equal if they represent identical types.
 type Type interface {
 	// Methods applicable to all types.