add note on multiselect listbox keyboard behavior

content/patterns/listbox/examples/listbox-rearrangeable.html

@@ -265,6 +265,9 @@ <h2 id="kbd_label">Keyboard Support</h2>
       </tbody>
     </table>
     <h4 id="kbd_label_multiselect">Multiple selection keys supported in example 2</h4>
+    <p class="note">
+      The selection behavior demonstrated differs from the behavior provided by browsers for native HTML <code>&lt;select multiple&gt;</code> elements. The select element behavior is to alter selection with unmodified up/down arrow keys, requiring the use of modifier keys to select multiple options. This example demonstrates a multiple selection pattern that does not require the use of modifier keys.
+    </p>
     <table aria-labelledby="kbd_label_multiselect" class="def">
       <thead>
         <tr>