Updated IID length text

[Brian E Carpenter <[e-mail]>]

OK, after all that discussion, here is a revised proposal. There is
nothing new here at all, IMHO; just clarification:

OLD
   For all unicast addresses, except those that start with the binary
   value 000, Interface IDs are required to be 64 bits long.  Background
   on the 64 bit boundary in IPv6 addresses can be found in [RFC7421].

NEW
   IPv6 routing is based on prefixes of any valid length up to 128 [BCP198].
   For example, [RFC6164] standardises 127 bit prefixes on point-to-point
   links. However, consistent use of Stateless Address Autoconfiguration
   (SLAAC)[RFC4862] requires that all interfaces on a link use the same length
   of Interface ID. To guarantee interoperability of SLAAC, a fixed length of
   Interface ID is necessary. For all currently allocated unicast addresses,
   except those that start with the binary value 000, Interface IDs are
   required to be 64 bits long.  Background on the 64 bit boundary in IPv6
   addresses can be found in [RFC7421].

Regards
   Brian