Correct documentation for `userDestinationPrefix` in `MessageBrokerRegistry`

[Xednar]

Changes documentation for setUserDestinationPrefix to match getTargetDestination in DefaultUserDestinationResolver

Current documentation is misleading, because the default implementation appends "-user" + sessionId to the actualDestination

@pivotal-cla This is an Obvious Fix

[rstoyanchev]

Actually, I think you're right on the format of the target destination. My apologies!