Mention 'void' as the proper return type when there's nothing to

return, per suggestion from Joachim Wieland.

doc/src/sgml/ref/create_function.sgml

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-$PostgreSQL: pgsql/doc/src/sgml/ref/create_function.sgml,v 1.68 2006/09/16 00:30:17 momjian Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/create_function.sgml,v 1.69 2006/11/03 07:17:36 tgl Exp $
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 <refentry id="SQL-CREATEFUNCTION">
@@ -153,6 +153,8 @@ CREATE [ OR REPLACE ] FUNCTION
        or may reference the type of a table column.
        Depending on the implementation language it may also be allowed
        to specify <quote>pseudotypes</> such as <type>cstring</>.
+       If the function is not supposed to return a value, specify
+       <type>void</> as the return type.
       </para>
       <para>
        When there are <literal>OUT</> or <literal>INOUT</> parameters,