pgsql: Add section to Migration talking about typical areas that change

Log Message:
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Add section to Migration talking about typical areas that change between
major releases.

Modified Files:
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pgsql/doc/src/sgml:
backup.sgml (r2.113 -> r2.114)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml?r1=2.113&r2=2.114)