pgsql: Add some notes about how pg_dump relates to the practices

Log Message:
-----------
Add some notes about how pg_dump relates to the practices recommended
under 'Populating a Database'.

Modified Files:
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pgsql/doc/src/sgml:
perform.sgml (r1.52 -> r1.53)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/perform.sgml.diff?r1=1.52&r2=1.53)