Include SpawnScene in docs for Main schedule (#21624)

# Objective

Currently the docs for
[`SpawnScene`](https://docs.rs/bevy/latest/bevy/app/struct.SpawnScene.html)
link to the
[`Main`](https://docs.rs/bevy/latest/bevy/prelude/struct.Main.html)
schedule, but those docs don't mention when it runs.

## Solution

Updates docs to include this information, based on the schedule ordering
defined here:


https://github.com/bevyengine/bevy/blob/78d940cbfe177e3585fe19145e73c76172f4085e/crates/bevy_app/src/main_schedule.rs#L222-L224

Author: andrewhickman

crates/bevy_app/src/main_schedule.rs

@@ -33,6 +33,7 @@ use bevy_ecs::{
 /// * [`RunFixedMainLoop`]
 ///     * This will run [`FixedMain`] zero to many times, based on how much time has elapsed.
 /// * [`Update`]
+/// * [`SpawnScene`]
 /// * [`PostUpdate`]
 /// * [`Last`]
 ///
@@ -173,7 +174,7 @@ pub struct Update;
 
 /// The schedule that contains scene spawning.
 ///
-/// See the [`Main`] schedule for some details about how schedules are run.
+/// This runs after [`Update`] and before [`PostUpdate`]. See the [`Main`] schedule for more details about how schedules are run.
 #[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash, Default)]
 pub struct SpawnScene;