Add method doc to Scene.RemoveClient() to ask any callers to use Scene.IncomingCloseAgent() instead.

IncomingCloseAgent() now sets the scene presence state machine properly, which is necessary to avoid races between multiple sources of close.
Hence, it's also necessary for everyone to consistently call IncomingCloseAgent()
Calling RemoveClient() directly is currently generating an attention-grabbing exception though this right now this is harmless.