Clarify distinction between single-user and multi-user sharing of signals

[Legioth]

https://github.com/vaadin/docs/blob/main/articles/flow/ui-state/shared-signals.adoc is confusing since the introduction is about the SharedXyzSignal types whereas some of the examples are for use cases that don't share state between users and should thus use ValueSignal instead. Furthermore, at least the SystemStatusSignal example is using ValueSignal even though it should probably use SharedValueSignal instead.