Support for message priority lacks documentation

Customers are complaining that JMS message priority does not work in a predictable way, especially when paging is in operation. For example, it's not clear how far back from the head of the queue a higher-priority message can be, to be delivered ahead of a lower-priority message (with and without paging). It's not clear whether message pre-fetch or link credit affects this operation, or whether it differs with different wire protocols. At present, it seems safest to assume that message priority is not supported at all; but it clearly does work in some scenarios. We just need to document what those scenarios are.