wayland: add a warning in the examples in case no window is displayed.

examples/multiwindow.rs

@@ -9,6 +9,12 @@ fn main() {
 
     let mut num_windows = 3;
 
+    if cfg!(target_os = "linux") {
+        println!("Running this example under wayland may not display a window at all.\n\
+                  This is normal and because this example does not actually draw anything in the window,\
+                  thus the compositor does not display it.");
+    }
+
     events_loop.run_forever(|event| {
         match event {
             winit::Event::WindowEvent { event: winit::WindowEvent::Closed, window_id } => {