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>

docs/templates/pages/less.mustache

@@ -793,6 +793,11 @@
         <td><code>@select</code></td>
         <td>{{_i}}Control cursor selection of text on a page{{/i}}</td>
       </tr>
+      <tr>
+        <td><code>.backface-visibility()</code></td>
+        <td><code>@visibility: visible</code></td>
+        <td>{{_i}}Prevent flickering of content when using CSS 3D transforms{{/i}}</td>
+      </tr>
       <tr>
         <td><code>.resizable()</code></td>
         <td><code>@direction: both</code></td>

less/mixins.less

@@ -251,9 +251,11 @@
           transform: translate(@x, @y, @z);
 }
 
-// Back-face Visibility
+// Backface visibility
-// This prevents the browser flickering when using to CSS transformations
+// Prevent browsers from flickering when using CSS 3D transforms.
-.backface-visibility(@visibility){
+// Default value is `visible`, but can be changed to `hidden
+// See git pull https://github.com/dannykeane/bootstrap.git backface-visibility for examples
+.backface-visibility(@visibility: visible){
 	-webkit-backface-visibility: @visibility;
 	   -moz-backface-visibility: @visibility;
 	    -ms-backface-visibility: @visibility;