pgsql: Update constraint_exclusion docs because parameter is now

Log Message:
-----------
Update constraint_exclusion docs because parameter is now 3-valued, not
just enable/disable.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.211 -> r1.212)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.211&r2=1.212)