Update: hr within select element

closes #480 

Outside of browsers / AT significantly changing how people interact with the UA-provided listboxs, I don't see how the use of hr can be more than declarative decoration, since the element will not be directly accessible to anyone using AT.

If anyone thinks otherwise, and browsers/AT are willing to make changes to how this component has been behaving/exposed, we can discuss and change the PR.

index.html

@@ -2792,6 +2792,7 @@ <h4 id=el-hr>`hr`</h4>
     <tr>
       <th>Comments</th>
       <td>
+        <p>If an `hr` element is a descendant of a `select` element, user agents MAY expose the element with a role of `none`.</p>
       </td>
     </tr>
   </tbody>