Correction for HTML element role mapping

per w3c/aria#2504 - the html element doesn't actually map to the `document` role - rather a parent `document node` to the `html` element is what exposes the `document` role of a web page.

This update fixes the spec to match the updated HTML aam mapping / matches reality.  

The `generic` role is being added as an "allowed" but not recommended role to the `html` element.  The `document` role is being kept as an allowed role for the `html` element.  The rational there being that this has been in the spec for a long time and even when specified on the html element, from what i can tell, there's no pro/con to its inclusion.

Author: scottaohara

index.html

@@ -1531,11 +1531,12 @@ <h2 id="docconformance">
               [^html^]
             </th>
             <td>
-              <code>role=<a href="#index-aria-document">document</a></code>
+              <code>role=<a href="#index-aria-generic">generic</a></code>
             </td>
             <td>
               <p>
-                <a><strong class="nosupport">No `role`</strong></a> other than <code><a href="#index-aria-document">document</a></code>, which is NOT RECOMMENDED.
+                <a><strong class="nosupport">No `role`</strong></a> other than <code><a href="#index-aria-document">document</a></code> 
+                or <code><a href="#index-aria-generic">generic</a></code>, which are NOT RECOMMENDED.
               </p>
               <p>
                 <strong>No `aria-*` attributes</strong>.