System.on() and System.off(); needs to be documented more completely

https://github.com/particle-iot/firmware/blob/develop/wiring/inc/spark_wiring_system.h#L219-L241

    static bool on(system_event_t events, void(*handler)(system_event_t, int,void*)) {
        return !system_subscribe_event(events, reinterpret_cast<system_event_handler_t*>(handler), nullptr);
    }

    static bool on(system_event_t events, void(*handler)(system_event_t, int)) {
        return system_subscribe_event(events, reinterpret_cast<system_event_handler_t*>(handler), NULL);
    }

    static bool on(system_event_t events, void(*handler)(system_event_t)) {
        return system_subscribe_event(events, reinterpret_cast<system_event_handler_t*>(handler), NULL);
    }

    static bool on(system_event_t events, void(*handler)()) {
        return system_subscribe_event(events, reinterpret_cast<system_event_handler_t*>(handler), NULL);
    }

    static void off(void(*handler)(system_event_t, int,void*)) {
        system_unsubscribe_event(all_events, handler, nullptr);
    }

    static void off(system_event_t events, void(*handler)(system_event_t, int,void*)) {
        system_unsubscribe_event(events, handler, nullptr);
    }

You can logically OR system events together with System.on(event1 | event2 | event3, handler);

To unregister a system handler, you can call System.off(handler) when you have previously called System.on(event1 | event2 | event3, handler); to unregister all events, or System.off(event3, handler); to unregister just event3

Note: event1,2,3 just temp names, refer to system events in docs for all available events.

Start with the overloads in wiring/inc/spark_wiring_system.h#L219-L241 and inspect the existing System event documentation. Document combining events with bitwise OR, the available on() handler forms, and how off() removes all or selected event subscriptions. Done means the documented behavior matches these overloads and examples.