Mention in .add_observer() docs that first parameter must be a Trigger (#19315)

# Objective

Fix https://github.com/bevyengine/bevy/issues/13860

## Solution

Add note in docs that Trigger must be the first parameter of observer
systems

crates/bevy_app/src/app.rs

@@ -1306,6 +1306,8 @@ impl App {
 
     /// Spawns an [`Observer`] entity, which will watch for and respond to the given event.
     ///
+    /// `observer` can be any system whose first parameter is a [`Trigger`].
+    ///
     /// # Examples
     ///
     /// ```rust
@@ -1326,7 +1328,7 @@ impl App {
     /// # #[derive(Component)]
     /// # struct Friend;
     /// #
-    /// // An observer system can be any system where the first parameter is a trigger
+    ///
     /// app.add_observer(|trigger: Trigger<Party>, friends: Query<Entity, With<Friend>>, mut commands: Commands| {
     ///     if trigger.event().friends_allowed {
     ///         for friend in friends.iter() {

crates/bevy_ecs/src/observer/mod.rs

@@ -536,6 +536,8 @@ impl World {
     /// Spawns a "global" [`Observer`] which will watch for the given event.
     /// Returns its [`Entity`] as a [`EntityWorldMut`].
     ///
+    /// `system` can be any system whose first parameter is a [`Trigger`].
+    ///
     /// **Calling [`observe`](EntityWorldMut::observe) on the returned
     /// [`EntityWorldMut`] will observe the observer itself, which you very
     /// likely do not want.**

crates/bevy_ecs/src/system/commands/mod.rs

@@ -1068,6 +1068,8 @@ impl<'w, 's> Commands<'w, 's> {
     /// Spawns an [`Observer`] and returns the [`EntityCommands`] associated
     /// with the entity that stores the observer.
     ///
+    /// `observer` can be any system whose first parameter is a [`Trigger`].
+    ///
     /// **Calling [`observe`](EntityCommands::observe) on the returned
     /// [`EntityCommands`] will observe the observer itself, which you very
     /// likely do not want.**
@@ -1075,6 +1077,8 @@ impl<'w, 's> Commands<'w, 's> {
     /// # Panics
     ///
     /// Panics if the given system is an exclusive system.
+    ///
+    /// [`Trigger`]: crate::observer::Trigger
     pub fn add_observer<E: Event, B: Bundle, M>(
         &mut self,
         observer: impl IntoObserverSystem<E, B, M>,