pgsql: Add mention that Shared Disk Failover can use file system

Log Message:
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Add mention that Shared Disk Failover can use file system mirroring of
the mirroring is consistent.

Modified Files:
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pgsql/doc/src/sgml:
high-availability.sgml (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/high-availability.sgml.diff?r1=1.2&r2=1.3)