Editorial: input type=image should match type=reset|submit (#508)

Editorial: input type=image should match type=reset|submit

The allowed roles were made consistent in an earlier PR, but the other accompanying 'not recommended' guidance was missed.

closes #504

expand on the 'if possible' paragraph to call out that it'd be better for authors to use the button element instead of these legacy (though still valid) input button types.

index.html

@@ -1826,7 +1826,7 @@ <h2 id="docconformance">
             <td>
               <div class="proposed addition">
                 <p>
-                  Roles:
+                  The following roles are allowed, but are NOT RECOMMENDED:
                   <a href="#index-aria-button">`button`</a>,
                   <a href="#index-aria-checkbox">`checkbox`</a>,
                   <a href="#index-aria-gridcell">`gridcell`</a>,
@@ -1846,6 +1846,10 @@ <h2 id="docconformance">
                   <a data-cite="wai-aria-1.2#global_states">Global `aria-*` attributes</a> 
                   and any `aria-*` attributes applicable to the allowed roles.
                 </p>
+                <p>
+                  If possible, authors SHOULD consider using a different HTML element which allows the specified role,
+                  such as the `button` element.
+                </p>
               </div>
             </td>
           </tr>
@@ -1981,7 +1985,8 @@ <h2 id="docconformance">
                   and any `aria-*` attributes applicable to the allowed roles.
                 </p>
                 <p>
-                  If possible, authors SHOULD consider using a different HTML element which allows the specified role.
+                  If possible, authors SHOULD consider using a different HTML element which allows the specified role,
+                  such as the `button` element.
                 </p>
               </div>
             </td>
@@ -2036,7 +2041,8 @@ <h2 id="docconformance">
                   and any `aria-*` attributes applicable to the allowed roles.
                 </p>
                 <p>
-                  If possible, authors SHOULD consider using a different HTML element which allows the specified role.
+                  If possible, authors SHOULD consider using a different HTML element which allows the specified role,
+                  such as the `button` element.
                 </p>
               </div>
             </td>