pgsql: Document that expression indexes that call functions allow

Log Message:
-----------
Document that expression indexes that call functions allow function-level
statistics.

Modified Files:
--------------
pgsql/doc/src/sgml:
maintenance.sgml (r1.99 -> r1.100)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/maintenance.sgml?r1=1.99&r2=1.100)