reflect: document that method sets are lexicographically sorted

Fixes #30688

Change-Id: I8b63feba4b18bc07a09f6fbfaa33c1b3326b40e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/169597Reviewed-by: Rob Pike <r@golang.org>

src/reflect/type.go

@@ -54,6 +54,9 @@ type Type interface {
 	//
 	// For an interface type, the returned Method's Type field gives the
 	// method signature, without a receiver, and the Func field is nil.
+	//
+	// Only exported methods are accessible and they are sorted in
+	// lexicographic order.
 	Method(int) Method
 
 	// MethodByName returns the method with that name in the type's