pgsql-server: Mention that transactions can complete in a different

Log Message:
-----------
Mention that transactions can complete in a different numeric order, for
PITR recovery.

Modified Files:
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pgsql-server/doc/src/sgml:
backup.sgml (r2.42 -> r2.43)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/backup.sgml.diff?r1=2.42&r2=2.43)