AppExit documentation updates (bevyengine#7067)

Help users understand how to write code that runs when the app is
exiting.

See:

- bevyengine#7067 (Partial resolution)

Added documentation to `AppExit` class that mentions using the `Drop`
trait for code that needs to run on program exit, as well as linking
to the caveat about `App::run()` not being guaranteed to return.

crates/bevy_app/src/app.rs

@@ -1166,7 +1166,11 @@ fn run_once(mut app: App) {
 ///
 /// You can also use this event to detect that an exit was requested. In order to receive it, systems
 /// subscribing to this event should run after it was emitted and before the schedule of the same
-/// frame is over.
+/// frame is over. This is important since [`App::run()`] might never return.
+///
+/// If you don't require access to other components or resources, consider implementing the [`Drop`]
+/// trait on components/resources for code that runs on exit. That saves you from worrying about
+/// system schedule ordering, and is idiomatic Rust.
 #[derive(Debug, Clone, Default)]
 pub struct AppExit;