pgsql: pg_buffercache needs to be taught about relation forks, as Greg

Log Message:
-----------
pg_buffercache needs to be taught about relation forks, as Greg Stark
pointed out.

Modified Files:
--------------
pgsql/contrib/pg_buffercache:
pg_buffercache.sql.in (r1.7 -> r1.8)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_buffercache/pg_buffercache.sql.in?r1=1.7&r2=1.8)
pg_buffercache_pages.c (r1.14 -> r1.15)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_buffercache/pg_buffercache_pages.c?r1=1.14&r2=1.15)
pgsql/doc/src/sgml:
pgbuffercache.sgml (r2.2 -> r2.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/pgbuffercache.sgml?r1=2.2&r2=2.3)