reflect: update NumMethod doc for interface type

Updates #42123

Change-Id: Ieb43b65c88d15b2475b6f3dd9672c44e7831cc34
Reviewed-on: https://go-review.googlesource.com/c/go/+/264357
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/reflect/type.go

@@ -50,13 +50,13 @@ type Type interface {
 	// It panics if i is not in the range [0, NumMethod()).
 	//
 	// For a non-interface type T or *T, the returned Method's Type and Func
-	// fields describe a function whose first argument is the receiver.
+	// fields describe a function whose first argument is the receiver,
+	// and only exported methods are accessible.
 	//
 	// 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.
+	// Methods are sorted in lexicographic order.
 	Method(int) Method
 
 	// MethodByName returns the method with that name in the type's
@@ -69,7 +69,9 @@ type Type interface {
 	// method signature, without a receiver, and the Func field is nil.
 	MethodByName(string) (Method, bool)
 
-	// NumMethod returns the number of exported methods in the type's method set.
+	// NumMethod returns the number of methods accessible using Method.
+	//
+	// Note that NumMethod counts unexported methods only for interface types.
 	NumMethod() int
 
 	// Name returns the type's name within its package for a defined type.