NEW ISSUE: RequestContext.getCallbackReference() description inadequate

Title: RequestContext.getCallbackReference() description inadequate

Description: RequestContext.getCallbackReference() description does not 
say what should happen if it is called for a callback request or if it 
called for a request whose interface is not bi-directional.

Proposal:
change the description to say that in the above situation it either MUST 
return a null or MUST throw an exception.