pgsql: Add description that WAL files used during backup have to be

Log Message:
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Add description that WAL files used during backup have to be archived
before you are done.

Modified Files:
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pgsql/doc/src/sgml:
backup.sgml (r2.62 -> r2.63)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml.diff?r1=2.62&r2=2.63)