document using ObserverState as filter for Observer Entitys (#14669)

# Objective

~~Enables writing queries like `Query<Entity, With<ObserverMarker>>` to
filter `Entity`s that are, or are not (with `Without`), `Observer`s.~~

~~`Observer` version of [similar
PR](https://github.com/bevyengine/bevy/pull/14584) for `SystemId`s.~~

just adding a line to the docs :)

## Solution

~~Simple unit struct `ObserverMarker` added in `Observer`'s `.on_add`
component hook.~~

## Testing

No tests, but happy to write some with direction.

crates/bevy_ecs/src/observer/runner.rs

@@ -257,7 +257,8 @@ pub type ObserverRunner = fn(DeferredWorld, ObserverTrigger, PtrMut, propagate:
 /// You can call [`Observer::watch_entity`] more than once, which allows you to watch multiple entities with the same [`Observer`].
 ///
 /// When first added, [`Observer`] will also create an [`ObserverState`] component, which registers the observer with the [`World`] and
-/// serves as the "source of truth" of the observer.
+/// serves as the "source of truth" of the observer. [`ObserverState`] can be used to filter for [`Observer`] [`Entity`]s, e.g.
+/// [`With<ObserverState>`].
 ///
 /// [`SystemParam`]: crate::system::SystemParam
 pub struct Observer<T: 'static, B: Bundle> {