Add default stability level to docs of exported things.

src/ol/source/xyzsource.js

@@ -13,6 +13,7 @@ goog.require('ol.tilegrid.XYZ');
  * @constructor
  * @extends {ol.source.TileImage}
  * @param {ol.source.XYZOptions} options XYZ options.
+ * @todo stability experimental
  */
 ol.source.XYZ = function(options) {
 
@@ -67,6 +68,7 @@ ol.source.XYZ.prototype.setTileUrlFunction = function(tileUrlFunction) {
 
 /**
  * @param {string} url URL.
+ * @todo stability experimental
  */
 ol.source.XYZ.prototype.setUrl = function(url) {
   this.setTileUrlFunction(ol.TileUrlFunction.createFromTemplates(