gesture: add comment to Scroll.Add

Provide a reference to the documentation of io/pointer.InputOp.

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

gesture/gesture.go

@@ -244,6 +244,8 @@ func (c *Click) Events(q event.Queue) []ClickEvent {
 func (ClickEvent) ImplementsEvent() {}
 
 // Add the handler to the operation list to receive scroll events.
+// The bounds variable refers to the scrolling boundaries
+// as defined in io/pointer.InputOp.
 func (s *Scroll) Add(ops *op.Ops, bounds image.Rectangle) {
 	oph := pointer.InputOp{
 		Tag:          s,