Also, added some explanation of how this will feed back to overall signalling
application behaviour (again in 5.3.3):

   When the rate limiter is in effect, D-mode messages MUST be queued
   until transmission is re-enabled, or they MAY be dropped with an
   error condition indicated back to local signalling applications.  In
   either case, the effect of this will be to reduce the rate at which
   new transactions can be initiated by signalling applications, thereby
   reducing the load on the network.