Make RouteScopeOwner for PreserveOnRefresh more discoverable

[mshabarov]

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I've heard few concerns about AtPreserveOnRefresh not working, when coupled with AtUIScoped beans.
Indeed this combo doesn't work because of architectural limitations, however, you can make this case work with using AtRouteScope + AtRouteScopeOwner + AtPreserveOnRefresh as documented here https://vaadin.com/docs/latest/flow/integrations/spring/scopes#routescope.preserve (same article is in CDI integration).

So the quick improvement is to describe in a human way this possibility in https://vaadin.com/docs/latest/flow/advanced/preserving-state-on-refresh. We may think of another better API, but that's another story.

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