Normative: Restrict completion values callables can return (#2448)

spec.html

@@ -3836,6 +3836,8 @@ <h1>The Completion Record Specification Type</h1>
         </table>
       </emu-table>
       <p>The term &ldquo;<dfn>abrupt completion</dfn>&rdquo; refers to any completion with a [[Type]] value other than ~normal~.</p>
+      <p>Callable objects that are defined in this specification only return a normal completion or a throw completion. Returning any other kind of completion is considered an editorial error.</p>
+      <p>Implementation-defined callable objects must return either a normal completion or a throw completion.</p>
 
       <emu-clause id="await" oldids="await-fulfilled,await-rejected" aoid="Await">
         <h1>Await</h1>