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/pltcl.sgml

@@ -186,6 +186,18 @@ $$ LANGUAGE pltcl;
 </programlisting>
     </para>
 
+    <para>
+     Output arguments of procedures are returned in the same way, for example:
+
+<programlisting>
+CREATE PROCEDURE tcl_triple(INOUT a integer, INOUT b integer) AS $$
+    return [list a [expr {$1 * 3}] b [expr {$2 * 3}]]
+$$ LANGUAGE pltcl;
+
+CALL tcl_triple(5, 10);
+</programlisting>
+    </para>
+
     <tip>
      <para>
       The result list can be made from an array representation of the