Doc patch to note which system catalogs have oids

Hi,

The attached patch documents the oid column of those
system catalogs having an oid.

Distinguish system catalogs with an oid from those without
and make the primary key clear to the newbie.

Found catalogs with an oid by querying a 9.2 installation:

select pg_class.relkind, pg_class.relname
from pg_class, pg_attribute
where pg_attribute.attrelid = pg_class.oid
and pg_attribute.attname = 'oid'
and pg_class.relname like 'pg_%'
and (pg_class.relkind = 'r' -- table
or pg_class.relkind = 'v') -- view
order by pg_class.relkind, pg_class.relname;

Karl <kop(at)meme(dot)com>
Free Software: "You don't pay back, you pay forward."
-- Robert A. Heinlein