Add and fix documentation for atlas manager

examples/symbol-atlas-webgl.html

@@ -34,6 +34,12 @@
           <h4 id="title">Symbols with WebGL example</h4>
           <p id="shortdesc">Using symbols in an atlas with WebGL.</p>
           <div id="docs">
+            <p>When using symbol styles with WebGL, OpenLayers would render the symbol
+            on a temporary image and would create a WebGL texture for each image. For a
+            better performance, it is recommended to use atlas images (similar to
+            image sprites with CSS), so that the number of textures is reduced. OpenLayers
+            provides an <code>AtlasManager</code>, which when passed to the constructor
+            of a symbol style, will create atlases for the symbols.</p>
             <p>See the <a href="symbol-atlas-webgl.js" target="_blank">symbol-atlas-webgl.js source</a> to see how this is done.</p>
           </div>
           <div id="tags">webgl, symbol, atlas, vector, point</div>