Modal Dialog Example: Add aria-modal and aria-hidden notes

For issue #480, per input from @DavidMacDonald,
modified examples/dialog-modal/dialog.html.
Following the roles, states, and properties table, added a subsection with 3 notes regarding aria-modal and aria-hidden.

examples/dialog-modal/dialog.html

@@ -178,6 +178,24 @@ <h2 id="rps_label">Role, Property, State, and Tabindex  Attributes</h2>
             </tr>
           </tbody>
         </table>
+        <h3>Notes on <code>aria-modal</code>and <code>aria-hidden</code></h3>
+        <ol>
+          <li>
+            The <code>aria-modal</code> property was introduced in ARIA 1.1.
+            As a new property, screen reader users may experience varying degrees of support for it.
+          </li>
+          <li>
+            Applying the <code>aria-modal</code> property to the <code>dialog</code> element 
+            replaces the technique of using <code>aria-hidden</code> on the background for informing assistive technologies that content outside a dialog is inert.
+          </li>
+          <li>
+            In legacy dialog implementations where <code>aria-hidden</code> is used to make content outside a dialog inert for assistive technology users, it is important that:
+            <ol>
+              <li><code>aria-hidden</code> is set to <code>true</code> on each element containing a portion of the inert layer.</li>
+              <li>The dialog element is not a descendant of any element that has <code>aria-hidden</code> set to <code>true</code>.</li>
+            </ol>
+          </li>
+        </ol>
       </section>
 
       <section>