document .backface-visibility and update comments and initial value

docs/less.html

@@ -870,6 +870,11 @@
         <td><code>@select</code></td>
         <td>Control cursor selection of text on a page</td>
       </tr>
+      <tr>
+        <td><code>.backface-visibility()</code></td>
+        <td><code>@visibility: visible</code></td>
+        <td>Prevent flickering of content when using CSS 3D transforms</td>
+      </tr>
       <tr>
         <td><code>.resizable()</code></td>
         <td><code>@direction: both</code></td>