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

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

## Solution
- 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

@@ -762,7 +762,8 @@ 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
@@ -805,7 +806,7 @@ 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;
 ///     }
@@ -813,7 +814,7 @@ 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_render/src/lib.rs

@@ -251,7 +251,7 @@ impl Plugin for RenderPlugin {
                 .add_systems(
                     Render,
                     (
-                        // This set applies the commands from the extract stage while the render schedule
+                        // This set applies the commands from the extract schedule while the render schedule
                         // is running in parallel with the main app.
                         apply_extract_commands.in_set(RenderSet::ExtractCommands),
                         (

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_transform/src/components/global_transform.rs

@@ -25,7 +25,7 @@ use bevy_reflect::{std_traits::ReflectDefault, FromReflect, Reflect};
 /// [`TransformPropagate`](crate::TransformSystem::TransformPropagate).
 ///
 /// This system runs during [`PostUpdate`](bevy_app::PostUpdate). If you
-/// update the [`Transform`] of an entity in this stage or after, you will notice a 1 frame lag
+/// update the [`Transform`] of an entity in this schedule or after, you will notice a 1 frame lag
 /// before the [`GlobalTransform`] is updated.
 ///
 /// # Examples

crates/bevy_transform/src/lib.rs

@@ -41,7 +41,7 @@ use systems::{propagate_transforms, sync_simple_transforms};
 /// [`TransformPropagate`](crate::TransformSystem::TransformPropagate).
 ///
 /// This system runs during [`PostUpdate`](bevy_app::PostUpdate). If you
-/// update the [`Transform`] of an entity in this stage or after, you will notice a 1 frame lag
+/// update the [`Transform`] of an entity in this schedule or after, you will notice a 1 frame lag
 /// before the [`GlobalTransform`] is updated.
 #[derive(Bundle, Clone, Copy, Debug, Default)]
 pub struct TransformBundle {
@@ -61,7 +61,7 @@ impl TransformBundle {
     /// Creates a new [`TransformBundle`] from a [`Transform`].
     ///
     /// This initializes [`GlobalTransform`] as identity, to be updated later by the
-    /// [`PostUpdate`](bevy_app::PostUpdate) stage.
+    /// [`PostUpdate`](bevy_app::PostUpdate) schedule.
     #[inline]
     pub const fn from_transform(transform: Transform) -> Self {
         TransformBundle {

crates/bevy_winit/src/lib.rs

@@ -153,7 +153,7 @@ 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,

examples/shader/post_process_pass.rs

@@ -176,7 +176,7 @@ impl Node for PostProcessNode {
 
         // The bind_group gets created each frame.
         //
-        // Normally, you would create a bind_group in the Queue stage, but this doesn't work with the post_process_write().
+        // Normally, you would create a bind_group in the Queue set, but this doesn't work with the post_process_write().
         // The reason it doesn't work is because each post_process_write will alternate the source/destination.
         // The only way to have the correct source/destination for the bind_group is to make sure you get it during the node execution.
         let bind_group = render_context