Expand description for XYZ source

src/ol/source/xyzsource.js

@@ -8,7 +8,20 @@ goog.require('ol.source.TileImage');
 
 /**
  * @classdesc
- * Layer source for tile data with URLs in a set XYZ format.
+ * Layer source for tile data with URLs in a set XYZ format that are
+ * defined in a URL template. By default, this follows the widely-used
+ * Google grid where `x` 0 and `y` 0 are in the top left. Grids like
+ * TMS where `x` 0 and `y` 0 are in the bottom left can be used by
+ * using the `{-y}` placeholder in the URL template, so long as the
+ * source does not have a custom tile grid. In this case,
+ * {@link ol.source.TileImage} can be used with a `tileUrlFunction`
+ * such as:
+ *
+ *  tileUrlFunction: function(coordinate) {
+ *    return 'http://mapserver.com/' + coordinate[0] + '/' +
+ *        coordinate[1] + '/' + coordinate[2] + '.png';
+ *    }
+ *
  *
  * @constructor
  * @extends {ol.source.TileImage}