pgsql: Add cross-reference from wal_level to hot_standby setting.

Log Message:
-----------
Add cross-reference from wal_level to hot_standby setting. Update
the PITR documentation to mention that you need to set wal_level to
'archive' or 'hot_standby', to enable WAL archiving. Per Simon's request.

Modified Files:
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    pgsql/doc/src/sgml:
        backup.sgml (r2.154 -> r2.155)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml?r1=2.154&r2=2.155)
        config.sgml (r1.274 -> r1.275)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.274&r2=1.275)