Log Message:
-----------
Mention that tables have a visibility map fork alongside the main fork
and FSM.
Modified Files:
--------------
pgsql/doc/src/sgml:
storage.sgml (r1.26 -> r1.27)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/storage.sgml?r1=1.26&r2=1.27)