On Web, never return a MonitorHandle (#3051)

src/window.rs

@@ -1050,6 +1050,7 @@ impl Window {
     /// - **iOS:** Can only be called on the main thread.
     /// - **Android / Orbital:** Will always return `None`.
     /// - **Wayland:** Can return `Borderless(None)` when there are no monitors.
+    /// - **Web:** Can only return `None` or `Borderless(None)`.
     #[inline]
     pub fn fullscreen(&self) -> Option<Fullscreen> {
         self.window