pgsql: Add a few current_* variable index entries to the docs.

Log Message:
-----------
Add a few current_* variable index entries to the docs.

Guillaume Lelarge

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.412 -> r1.413)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.412&r2=1.413)