The unused_oids script should have a reminder to use the 8000-8999OID range

I pushed a commit that required a new pg_proc entry today. Had I not
been involved with the work that became commit a6417078, I would
definitely not have used an OID from the range reserved for devel
system catalogs (8000 - 8999). As I understand it, this is now
standard practice.

Perhaps unsurprisingly, other committers didn't get the memo, and
haven't been using the special reserved range since its introduction
in March. I think that this could be avoided by simply making
unused_oids print a reminder about the new practice.

Is it within the discretion of committers to not use the reserved
range? It seems preferable for everybody to consistently use the
reserved OID range.
-- 
Peter Geoghegan