Combobox Pattern: Revise Escape behavior (pull #1129)

For issue #1066, revises description of escape behavior.
Escape may optionally clear the text box,
but only if the popup is hidden before escape is pressed.
Previously, the pattern implied both could happen with a single press of escape, which is inconsistent with any known implementations.

aria-practices.html

@@ -809,7 +809,7 @@ <h5>Textbox Keyboard Interaction</h5>
             </ul>
           </li>
           <li><kbd>Up Arrow</kbd> (Optional): If the popup is available, places focus on the last focusable element in the popup.</li>
-          <li><kbd>Escape</kbd>: Dismisses the popup if it is visible. Optionally, clears the textbox.</li>
+          <li><kbd>Escape</kbd>: Dismisses the popup if it is visible. Optionally, if the popup is hidden before <kbd>Escape</kbd> is pressed, clears the textbox.</li>
           <li>
             <kbd>Enter</kbd>: If an autocomplete suggestion is automatically selected, accepts the suggestion either by placing the input cursor at the end of the accepted value in the textbox or by performing a default action on the value.
             For example, in a messaging application, the default action may be to add the accepted value to a list of message recipients and then clear the textbox so the user can add another recipient.