Explain EventWriter limits concurrency (bevyengine#11063)

Co-authored-by: François <[email protected]>
Co-authored-by: James Liu <[email protected]>

crates/bevy_ecs/src/event.rs

@@ -407,6 +407,11 @@ impl<E: Event> DerefMut for EventSequence<E> {
 }
 
 /// Reads events of type `T` in order and tracks which events have already been read.
+///
+/// # Concurrency
+///
+/// Unlike [`EventWriter<T>`], systems with `EventReader<T>` param can be executed concurrently
+/// (but not concurrently with `EventWriter<T>` systems for the same event type).
 #[derive(SystemParam, Debug)]
 pub struct EventReader<'w, 's, E: Event> {
     reader: Local<'s, ManualEventReader<E>>,
@@ -484,7 +489,12 @@ impl<'w, 's, E: Event> EventReader<'w, 's, E> {
 /// # bevy_ecs::system::assert_is_system(my_system);
 /// ```
 ///
-/// # Limitations
+/// # Concurrency
+///
+/// `EventWriter` param has [`ResMut<Events<T>>`](Events) inside. So two systems declaring `EventWriter<T>` params
+/// for the same event type won't be executed concurrently.
+///
+/// # Untyped events
 ///
 /// `EventWriter` can only send events of one specific type, which must be known at compile-time.
 /// This is not a problem most of the time, but you may find a situation where you cannot know