update docs

docs/assets/plugins/metadata/metadata.js

@@ -61,11 +61,11 @@
           <tr>
             <td class="nowrap"><code>updateComplete</code></td>
             <td>
-              A promise that resolves when the component has
+              A read-only promise that resolves when the component has
               <a href="/getting-started/usage?id=component-rendering-and-updating">finished updating</a>.
             </td>
             <td></td>
-            <td><code>Promise&lt;Boolean&gt;</td>
+            <td></td>
             <td></td>
           </tr>
         </tbody>