docs: update docs and comments that still refer to stages (#8156)

Documentation should no longer be using pre-stageless terminology to
avoid confusion.

- update all docs referring to stages to instead refer to sets/schedules
where appropriate
- also mention `apply_system_buffers` for anything system-buffer-related
that previously referred to buffers being applied "at the end of a
stage"

crates/bevy_ecs/examples/resources.rs

@@ -11,7 +11,7 @@ fn main() {
     // Add the counter resource
     world.insert_resource(Counter { value: 0 });
 
-    // Create a schedule and a stage
+    // Create a schedule
     let mut schedule = Schedule::default();
 
     // Add systems to increase the counter and to print out the current value

crates/bevy_ecs/src/event.rs

@@ -283,8 +283,8 @@ impl<'a, 'w, 's, E: Event> IntoIterator for &'a mut EventReader<'w, 's, E> {
 ///     // which allows one to do all kinds of clever things with type erasure, such as sending
 ///     // custom events to unknown 3rd party plugins (modding API).
 ///     //
-///     // NOTE: the event won't actually be sent until commands get flushed
-///     // at the end of the current stage.
+///     // NOTE: the event won't actually be sent until commands get applied during
+///     // apply_system_buffers.
 ///     commands.add(|w: &mut World| {
 ///         w.send_event(MyEvent);
 ///     });

crates/bevy_ecs/src/system/system_param.rs

@@ -769,7 +769,8 @@ pub trait SystemBuffer: FromWorld + Send + 'static {
     fn apply(&mut self, system_meta: &SystemMeta, world: &mut World);
 }
 
-/// A [`SystemParam`] that stores a buffer which gets applied to the [`World`] at the end of a stage.
+/// A [`SystemParam`] that stores a buffer which gets applied to the [`World`] during
+/// [`apply_system_buffers`](crate::schedule::apply_system_buffers).
 /// This is used internally by [`Commands`] to defer `World` mutations.
 ///
 /// [`Commands`]: crate::system::Commands
@@ -812,7 +813,7 @@ pub trait SystemBuffer: FromWorld + Send + 'static {
 /// struct AlarmFlag(bool);
 ///
 /// impl AlarmFlag {
-///     /// Sounds the alarm at the end of the current stage.
+///     /// Sounds the alarm the next time buffers are applied via apply_system_buffers.
 ///     pub fn flag(&mut self) {
 ///         self.0 = true;
 ///     }
@@ -820,7 +821,7 @@ pub trait SystemBuffer: FromWorld + Send + 'static {
 ///
 /// impl SystemBuffer for AlarmFlag {
 ///     // When `AlarmFlag` is used in a system, this function will get
-///     // called at the end of the system's stage.
+///     // called the next time buffers are applied via apply_system_buffers.
 ///     fn apply(&mut self, system_meta: &SystemMeta, world: &mut World) {
 ///         if self.0 {
 ///             world.resource_mut::<Alarm>().0 = true;

crates/bevy_transform/src/commands.rs

@@ -73,7 +73,8 @@ pub trait BuildChildrenTransformExt {
     /// [`Transform`].
     ///
     /// Note that both the hierarchy and transform updates will only execute
-    /// at the end of the current stage.
+    /// the next time commands are applied
+    /// (during [`apply_system_buffers`](bevy_ecs::schedule::apply_system_buffers)).
     fn set_parent_in_place(&mut self, parent: Entity) -> &mut Self;
 
     /// Make this entity parentless while preserving this entity's [`GlobalTransform`]
@@ -83,7 +84,8 @@ pub trait BuildChildrenTransformExt {
     /// [`Transform`].
     ///
     /// Note that both the hierarchy and transform updates will only execute
-    /// at the end of the current stage.
+    /// the next time commands are applied
+    /// (during [`apply_system_buffers`](bevy_ecs::schedule::apply_system_buffers)).
     fn remove_parent_in_place(&mut self) -> &mut Self;
 }
 impl<'w, 's, 'a> BuildChildrenTransformExt for EntityCommands<'w, 's, 'a> {

crates/bevy_winit/src/lib.rs

@@ -144,7 +144,7 @@ impl Plugin for WinitPlugin {
             ) = create_window_system_state.get_mut(&mut app.world);
 
             // Here we need to create a winit-window and give it a WindowHandle which the renderer can use.
-            // It needs to be spawned before the start of the startup-stage, so we cannot use a regular system.
+            // It needs to be spawned before the start of the startup schedule, so we cannot use a regular system.
             // Instead we need to create the window and spawn it using direct world access
             create_window(
                 commands,