Explain message and messageerror for ServiceWorkerContainer. (#1572)

Addresses #1549

Co-authored-by: Jake Archibald <[email protected]>

docs/index.bs

@@ -803,11 +803,11 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
           </tr>
           <tr>
             <td><dfn attribute for="ServiceWorkerContainer"><code>onmessage</code></dfn></td>
-            <td>{{message!!event}}</td>
+            <td>{{ServiceWorkerContainer/message!!event}}</td>
           </tr>
           <tr>
             <td><dfn attribute for="ServiceWorkerContainer"><code>onmessageerror</code></dfn></td>
-            <td>{{messageerror!!event}}</td>
+            <td>{{ServiceWorkerContainer/messageerror!!event}}</td>
           </tr>
         </tbody>
       </table>
@@ -872,6 +872,16 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
           <td>{{Event}}</td>
           <td>The [=ServiceWorkerContainer/service worker client=]'s <a>active service worker</a> changes. (See step 9.2 of the <a>Activate</a> algorithm. The <a>skip waiting flag</a> of a [=/service worker=] causes <a lt="activate">activation</a> of the [=/service worker registration=] to occur while [=/service worker clients=] are <a>using</a> the [=/service worker registration=], {{ServiceWorkerContainer/controller|navigator.serviceWorker.controller}} immediately reflects the <a>active worker</a> as the [=/service worker=] that <a>controls</a> the [=/service worker client=].)</td>
         </tr>
+        <tr>
+          <td><dfn event id="service-worker-container-message-event"><code>message</code></dfn></td>
+          <td>{{Event}}</td>
+          <td>The [=ServiceWorkerContainer/service worker client=] receives a message from a [=/service worker=]. See {{Client/postMessage(message, options)}}.</td>
+        </tr>
+        <tr>
+          <td><dfn event id="service-worker-container-messageerror-event"><code>messageerror</code></dfn></td>
+          <td>{{Event}}</td>
+          <td>The [=ServiceWorkerContainer/service worker client=] is sent a message that cannot be deserialized from a [=/service worker=]. See {{Client/postMessage(message, options)}}.</td>
+        </tr>
       </tbody>
     </table>
   </section>