Change PROCEDURE to FUNCTION in CREATE TRIGGER syntax

Since procedures are now a different thing from functions, change the
CREATE TRIGGER and CREATE EVENT TRIGGER syntax to use FUNCTION in the
clause that specifies the function.  PROCEDURE is still accepted for
compatibility.

pg_dump and ruleutils.c output is not changed yet, because that would
require a change in information_schema.sql and thus a catversion change.

Reported-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Jonathan S. Katz <jonathan.katz@excoventures.com>

doc/src/sgml/plperl.sgml

@@ -1300,7 +1300,7 @@ $$ LANGUAGE plperl;
 
 CREATE TRIGGER test_valid_id_trig
     BEFORE INSERT OR UPDATE ON test
-    FOR EACH ROW EXECUTE PROCEDURE valid_id();
+    FOR EACH ROW EXECUTE FUNCTION valid_id();
 </programlisting>
   </para>
  </sect1>
@@ -1350,7 +1350,7 @@ $$ LANGUAGE plperl;
 
 CREATE EVENT TRIGGER perl_a_snitch
     ON ddl_command_start
-    EXECUTE PROCEDURE perlsnitch();
+    EXECUTE FUNCTION perlsnitch();
 </programlisting>
   </para>
  </sect1>