api return restriction

docs/wiki/about-townengine.html

@@ -73,6 +73,7 @@
       <li>Procedure can't have more than 8 parameters.
       <li>Decimal portions of floating points are lossy both due to rounding errors and text representation,
           thus they cannot be relied to hold for equality. Integer parts of floats are good up to 2^24.
+      <li>Returns must be restricted/closed on being constant over frame, parameters or asset frame.
       </ul>
     </blockquote>
   </body>