pgsql: Adjust the discussion of triggers to more clearly guide people in

Log Message:
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Adjust the discussion of triggers to more clearly guide people in the
direction of writing triggers in a procedural language, rather than C.
Per discussion.

Modified Files:
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pgsql/doc/src/sgml:
trigger.sgml (r1.43 -> r1.44)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/trigger.sgml.diff?r1=1.43&r2=1.44)