reflect: document that value.NumMethod counts exported methods

Updates #17686
Fixes #20848

Change-Id: I35d58c7d1aa74d3e7867124070e27c787d444b04
Reviewed-on: https://go-review.googlesource.com/47210Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/reflect/value.go

@@ -1134,7 +1134,7 @@ func (v Value) Method(i int) Value {
 	return Value{v.typ, v.ptr, fl}
 }
 
-// NumMethod returns the number of methods in the value's method set.
+// NumMethod returns the number of exported methods in the value's method set.
 func (v Value) NumMethod() int {
 	if v.typ == nil {
 		panic(&ValueError{"reflect.Value.NumMethod", Invalid})