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/event-trigger.sgml

@@ -1044,7 +1044,7 @@ CREATE FUNCTION noddl() RETURNS event_trigger
     AS 'noddl' LANGUAGE C;
 
 CREATE EVENT TRIGGER noddl ON ddl_command_start
-    EXECUTE PROCEDURE noddl();
+    EXECUTE FUNCTION noddl();
 </programlisting>
    </para>
 
@@ -1129,7 +1129,7 @@ $$;
 
 CREATE EVENT TRIGGER no_rewrite_allowed
                   ON table_rewrite
-   EXECUTE PROCEDURE no_rewrite();
+   EXECUTE FUNCTION no_rewrite();
 </programlisting>
    </para>
  </sect1>