Make sample code complete

src/sdl2/event.rs

@@ -242,6 +242,8 @@ impl crate::EventSubsystem {
     ///
     /// # Example: dump every event to stderr
     /// ```
+    /// // `let _ = ...` is insufficient, as it is dropped immediately.
+    /// // You have to name the variable.
     /// let _event_watch = event_subsystem.add_event_watch(|event| {
     ///     dbg!(event);
     /// });