util/eventbus: flesh out docs a bit

Updates #cleanup

Change-Id: Ia6b0e4b0426be1dd10a777aff0a81d4dd6b69b01
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/eventbus/subscribe.go

@@ -158,7 +158,7 @@ func (q *subscribeState) subscriberFor(val any) subscriber {
 	return q.outputs[reflect.TypeOf(val)]
 }
 
-// Close closes the subscribeState. Implicitly closes all Subscribers
+// Close closes the subscribeState. It implicitly closes all Subscribers
 // linked to this state, and any pending events are discarded.
 func (s *subscribeState) close() {
 	s.dispatcher.StopAndWait()
@@ -244,6 +244,10 @@ func (s *Subscriber[T]) Done() <-chan struct{} {
 // Close closes the Subscriber, indicating the caller no longer wishes
 // to receive this event type. After Close, receives on
 // [Subscriber.Events] block for ever.
+//
+// If the Bus from which the Subscriber was created is closed,
+// the Subscriber is implicitly closed and does not need to be closed
+// separately.
 func (s *Subscriber[T]) Close() {
 	s.stop.Stop() // unblock receivers
 	s.unregister()