Improve description of snd-settle-mode and rcv-settle-mode

From:Steve Huston's comments:

Part 2, section 7.3, snd-settle-mode, rcv-settle-mode: What is the effect
of a Sender/Receiver not honoring the requested mode? Since there's only
one attach sent in each direction, what's the intended use of this? Only
one side can learn the other's preference before sending its own attach.

Also, need to clarify what happens if you do not hour the negotiated mode (e.g., send unsettled when you have stated you will only send settled)?