pgsql-server: Add a note suggesting that users should use the newer

Log Message:
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Add a note suggesting that users should use the newer version of pg_dump
to perform upgrades, and cleanup some nearby text. Patch from Robert
Treat, editorializing by Neil Conway.

Modified Files:
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pgsql-server/doc/src/sgml:
backup.sgml (r2.47 -> r2.48)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/backup.sgml.diff?r1=2.47&r2=2.48)