reflect: clarify that NumMethod returns only the number of exported methods

Fixes #17686.

Change-Id: I7d07c367e50b448579f9855bea43df76ddb82bd0
Reviewed-on: https://go-review.googlesource.com/32420Reviewed-by: Rob Pike <r@golang.org>

src/reflect/type.go

@@ -63,7 +63,7 @@ type Type interface {
 	// method signature, without a receiver, and the Func field is nil.
 	MethodByName(string) (Method, bool)
 
-	// NumMethod returns the number of methods in the type's method set.
+	// NumMethod returns the number of exported methods in the type's method set.
 	NumMethod() int
 
 	// Name returns the type's name within its package.