Add API documentation to PassiveSynchronizationChannel

[myronkscott]

There have been several conceptual problems with the way PassiveSchronizationChannel works within the API. Messages sent on this channel are sent inline to the passive as part of a linear message stream for a particular concurrency key. Messages sent using messageSelf will be added to the queue for the concurrency key at the end of the queue. This message can straddle both sides of the sync operation so care needs to be taken understand what this means.