pgsql: Clarify example of planner cost computation, per a suggestion

Log Message:
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Clarify example of planner cost computation, per a suggestion from
James Shaw. Also update a couple of examples to reflect 8.3's improved
plan-printing code.

Modified Files:
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pgsql/doc/src/sgml:
perform.sgml (r1.65 -> r1.66)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/perform.sgml?r1=1.65&r2=1.66)