Add docs

Author: CatThingy

crates/bevy_ecs/src/event.rs

@@ -11,6 +11,7 @@ use std::{fmt, hash::Hash, iter::Chain, marker::PhantomData, slice::Iter};
 ///
 /// Events must be thread-safe.
 pub trait Event: Send + Sync + 'static {
+    /// Whether a warning is emitted if an event is sent but never read
     const WARN_MISSED: bool = false;
 }
 
@@ -126,6 +127,15 @@ struct EventInstance<E: Event> {
 /// [Example usage.](https://github.com/bevyengine/bevy/blob/latest/examples/ecs/event.rs)
 /// [Example usage standalone.](https://github.com/bevyengine/bevy/blob/latest/crates/bevy_ecs/examples/events.rs)
 ///
+/// # Missed events
+///
+/// Obscure bugs may arise if an [`EventReader`] does not read all events before they are cleared. 
+/// To emit a warning when this happens, annotate the event with `#[event(warn_missed)]`.
+/// ```
+/// #[derive(Event)]
+/// #[event(warn_missed)]
+/// struct MyEvent;
+/// ```
 #[derive(Debug, Resource)]
 pub struct Events<E: Event> {
     /// Holds the oldest still active events.