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?