pgsql: Add documentation about when trigger values NEW/OLD return NULL.

Log Message:
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Add documentation about when trigger values NEW/OLD return NULL.

Jeff Davis

Modified Files:
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pgsql/doc/src/sgml:
plpgsql.sgml (r1.133 -> r1.134)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml?r1=1.133&r2=1.134)