pgsql: Add documentation mention of 'check_postgres.pl' in Routine

Log Message:
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Add documentation mention of 'check_postgres.pl' in Routine Database
Maintenance Tasks section.

Modified Files:
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pgsql/doc/src/sgml:
maintenance.sgml (r1.90 -> r1.91)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/maintenance.sgml?r1=1.90&r2=1.91)