pgsql: information_schema.key_column_usage.position_in_unique_constraint

Log Message:
-----------
information_schema.key_column_usage.position_in_unique_constraint was
misdocumented as not being implemented. In reality it has worked since
the release of 8.2.

Modified Files:
--------------
pgsql/doc/src/sgml:
information_schema.sgml (r1.34 -> r1.35)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/information_schema.sgml?r1=1.34&r2=1.35)