Allow comboboxes to invoke menus

closes #2050

This PR simply adds `menu` to the paragraph of text that indicates what value needs to be used for the aria-haspopup attribute, if the popup is not a listbox:

>authors MUST specify an aria-haspopup value of tree, grid, or dialog that corresponds to the role of its popup.

I think some additional text could be written in the `menu` role definition to further acknowledge this new allowance.  But I'll leave that open for discussion, rather than immediately start adding such text.

Author: scottaohara

index.html

@@ -2802,7 +2802,7 @@ <h2>Definition of Roles</h2>
             </p>
             <p>
               Elements with the role <code>combobox</code> have an implicit <pref>aria-haspopup</pref> value of <code>listbox</code>. If the <code>combobox</code> popup element has a role other than
-              <rref>listbox</rref>, authors MUST specify an <pref>aria-haspopup</pref> value of <rref>tree</rref>, <rref>grid</rref>, or <rref>dialog</rref> that corresponds to the role of its popup.
+              <rref>listbox</rref>, authors MUST specify an <pref>aria-haspopup</pref> value of <rref>tree</rref>, <rref>grid</rref>, <rref>menu</rref> or <rref>dialog</rref> that corresponds to the role of its popup.
             </p>
             <p>
               If the user interface includes an additional icon that allows the visibility of the popup to be controlled via pointer and touch events, authors SHOULD ensure that element has role