Document stipulations on usage of state during routing

[ianterrell]

Since routing through an app can be asynchronous in order to wait for animations, the state may have changed since the route was initiated.

So, for instance, in NavigationControllerMetaCoordinator's call func coordinators(for route: Route) -> [AnyCoordinator], it is not possible to rely on the state in order to know what coordinators to create. It must be done from the route alone.

This has an impact on how your state should be created.