doc improvement for StateTransition (#19597)

Fixes #19594
The exact problem is described in that issue.
I improved the docs to guide anyone who has the the same issue I had.
I kept myself minimal, since the problem is relatively niche, hopefully
it will be enough if anyone else has that problem

Author: Wuketuke

crates/bevy_state/src/state/transitions.rs

@@ -50,6 +50,8 @@ pub struct OnTransition<S: States> {
 /// }
 /// ```
 ///
+/// This schedule is split up into four phases, as described in [`StateTransitionSteps`].
+///
 /// [`PreStartup`]: https://docs.rs/bevy/latest/bevy/prelude/struct.PreStartup.html
 /// [`PreUpdate`]: https://docs.rs/bevy/latest/bevy/prelude/struct.PreUpdate.html
 #[derive(ScheduleLabel, Clone, Debug, PartialEq, Eq, Hash, Default)]