Allow aria-valuetext for use with combobox role

closes #2382
and is related to #2344 / #2369

This PR adds aria-valuetext as a supported property for the combobox role.

the `aria-valuetext` definition is slightly updated to indicate that the attribute can be used on other supported roles.  I'm not sure if we want to do any further updates to that section in this PR - or if that really should be handled in a larger PR to resolve #711

Author: scottaohara

index.html

@@ -2843,6 +2843,7 @@ <h2>Definition of Roles</h2>
               following:
             </p>
             <ul>
+              <li>If the <code>aria-valuetext</code> attribute is specified on the element with the <code>combobox</code> role, the value of the attribute is the value of the <code>combobox</code> element.</li>
               <li>
                 If the <code>combobox</code> element is a host language element that provides a value, such as an HTML <code>input</code> element, the value of the combobox is the value of that
                 element.
@@ -2949,6 +2950,7 @@ <h2>Definition of Roles</h2>
                     <li><sref>aria-invalid</sref></li>
                     <li><pref>aria-readonly</pref></li>
                     <li><pref>aria-required</pref></li>
+                    <li><pref>aria-valuetext</pref></li>
                   </ul>
                 </td>
               </tr>
@@ -16012,7 +16014,7 @@ <h2>Definitions of States and Properties (all aria-* attributes)</h2>
         <div class="property" id="aria-valuetext">
           <pdef>aria-valuetext</pdef>
           <div class="property-description">
-            <p><a>Defines</a> the human readable text alternative of <pref>aria-valuenow</pref> for a range <a>widget</a>.</p>
+            <p><a>Defines</a> the human readable text alternative of <pref>aria-valuenow</pref> for a range <a>widget</a>, or other widget roles which support the attribute.</p>
             <p>This property is used, for example, on a range widget such as a slider or progress bar.</p>
             <p>
               If the <pref>aria-valuetext</pref> attribute is set, authors SHOULD also set the <pref>aria-valuenow</pref> attribute, unless that value is unknown (for example, on an indeterminate