pgsql: Document use of rsync for file system backups.

Log Message:
-----------
Document use of rsync for file system backups.

Tino Wildenhain

Modified Files:
--------------
pgsql/doc/src/sgml:
backup.sgml (r2.56 -> r2.57)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml.diff?r1=2.56&r2=2.57)