Added the following text at the end of the MA-Hello description:

                                          GIST MAY use an MA-Hello
   request/response exchange on an existing association to verify that
   messages sent on it have reached the peer.  GIST MAY use the same
   technique to test the liveness of the underlying MA protocols
   themselves at arbitrary times.

Note that the MA-Hello request/response mechanism can be used to detect MA
protocol failure (faster than it might otherwise be reported).

NB the MA-Hello request/response is already described as a general purpose
diagnostic; this is just highlighting one use of it.