pgsql: Document that continuous archiving backup can be used for cases

Log Message:
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Document that continuous archiving backup can be used for cases where
you can't get a simultaneous snapshot.

Modified Files:
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pgsql/doc/src/sgml:
backup.sgml (r2.117 -> r2.118)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml?r1=2.117&r2=2.118)