unused_oids script should advertise reserved OID range

The unused_oids script has gone from being something of interest to
everybody that wants to write a patch that creates a new catalog
entry, to something that patch authors could do without in many cases.
I think that its output should prominently advertise that patches
should use random OIDs in the range 8000 - 9999. Commit
a6417078c4140e51cfd717448430f274b449d687 established that this should
be standard practice for patch authors.

Actually, maybe it should even suggest a *particular* random OID in
that range, so that the choice of OID is reliably random -- why even
require patch authors to pick a number at random?

It also looks like pg_proc.dat should be updated, since it still
mentions the old custom of trying to use contiguous OIDs. It also
discourages the practice of picking OIDs at random, which is almost
the opposite of what it should say.

--
Peter Geoghegan