Some explanation to the icon-sprite-webgl example

examples/icon-sprite-webgl.html

@@ -35,6 +35,7 @@
           <p id="shortdesc">Icon sprite with WebGL.</p>
           <div id="docs">
             <p>See the <a href="icon-sprite-webgl.js" target="_blank">icon-sprite-webgl.js source</a> to see how this is done.</p>
+            <p>In this example a sprite image is used for the icon styles. Using a sprite is required to get good performances with WebGL.</p>
           </div>
           <div id="tags">webgl, icon, sprite, vector, point</div>
         </div>