[html-aam] update ia2/atk mappings for maxlength attr

Supersedes [HTML AAM PR 522](w3c/html-aam#522)

Co-authored by: @smhigley

Author: scottaohara

html-aam/index.html

@@ -12044,7 +12044,9 @@ <h4 id="att-maxlength">`maxlength`</h4>
                 <a href="https://msdn.microsoft.com/en-us/library/dd373608%28v=VS.85%29.aspx">MSAA</a> + <a href="http://accessibility.linuxfoundation.org/a11yspecs/ia2/docs/html/">IAccessible2</a>
               </th>
               <td>
-                <div class="general">Not mapped</div>
+                <div class="objattrs">
+                  <span class="type">Object attributes:</span> `maxlength: &lt;value&gt;`
+                </div>
               </td>
             </tr>
             <tr>
@@ -12056,7 +12058,9 @@ <h4 id="att-maxlength">`maxlength`</h4>
             <tr>
               <th><a href="https://gnome.pages.gitlab.gnome.org/atk/">ATK</a></th>
               <td>
-                <div class="general">Not mapped</div>
+                <div class="objattrs">
+                  <span class="type">Object attributes:</span> `maxlength: &lt;value&gt;`
+                </div>
               </td>
             </tr>
             <tr>