Relationship in in/out/request/reply

In the WSDL binding, the "Relationship" refprop for the reply message is 
defined as consisting of (reply-to, request message ID).  We should at 
least clarify this to say that the relationship property MUST /include/ 
this pair, but may include others, analogous to the boilerplate about 
being able to include non-required properties as part of a larger 
conversation.

We should also clarify whether any (non-required) entries in the request 
message's Relationship refprop MUST or SHOULD be echoed back in the 
reply message's Relationship refprop.