Support INOUT arguments in procedures

In a top-level CALL, the values of INOUT arguments will be returned as a
result row.  In PL/pgSQL, the values are assigned back to the input
arguments.  In other languages, the same convention as for return a
record from a function is used.  That does not require any code changes
in the PL implementations.

Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>

doc/src/sgml/plpython.sgml

@@ -649,6 +649,17 @@ return (1, 2)
 $$ LANGUAGE plpythonu;
 
 SELECT * FROM multiout_simple();
+</programlisting>
+   </para>
+
+   <para>
+    Output parameters of procedures are passed back the same way.  For example:
+<programlisting>
+CREATE PROCEDURE python_triple(INOUT a integer, INOUT b integer) AS $$
+return (a * 3, b * 3)
+$$ LANGUAGE plpythonu;
+
+CALL python_triple(5, 10);
 </programlisting>
    </para>
   </sect2>