Added the following updated paragraph at the start of section 9 (IANA
considerations):

   The registry definitions that follow leave large blocks of codes marked
   "Reserved - not to be allocated".  This is to allow a future revision
   of this specification or another Standards Track document to modify
   the relative space given to different allocation policies without
   having to change the initial rules retrospectively if they turn out
   to have been inappropriate, e.g. if the space for one particular
   policy is exhausted too quickly.

The meaning of the word 'reserved' is apparently unclear, since it doesn't match
of the standard 2434 policies and might leave IANA unclear about what to do.

(The intention of the text is that this part of the registry is not open for
allocations under any policy. That needs to be made clearer.)