Include SpawnScene in docs for Main schedule

[andrewhickman]

Objective

Currently the docs for SpawnScene link to the Main schedule, but those docs don't mention when it runs.

Solution

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

bevy/crates/bevy_app/src/main_schedule.rs

Lines 222 to 224 in 78d940c

	Update.intern(),
	SpawnScene.intern(),
	PostUpdate.intern(),

[alice-i-cecile]

A bit iffy on this: it's added by a dependency, so it won't always exist and the doc link will be broken.

Can we instead add docs to SpawnScene clarifying exactly when it runs?

[andrewhickman]

I've added a note to SpawnScene.

I've left the link in Main for now, because currently SpawnScene is defined in the same crate as Main, so I think the link will always be valid. Happy to remove it if you still think that's best.

There are already several schedules in the Main docs from other crates, which are using direct links to docs.rs. Maybe it would make sense to follow that pattern, although we then wouldn't get an "unresolved link" error from rustdoc if it were moved to somewhere else like bevy_scene in future.