pgsql: Update PL/pgSQL trigger example to be clearer about how to

Log Message:
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Update PL/pgSQL trigger example to be clearer about how to "merge" data
into a table.

Jim C. Nasby

Modified Files:
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pgsql/doc/src/sgml:
plpgsql.sgml (r1.83 -> r1.84)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.83&r2=1.84)