Add deprecation notes for ol.Attribution

src/ol/attribution.js

@@ -22,6 +22,7 @@ goog.require('ol.tilegrid');
  *     ..
  *
  * @constructor
+ * @deprecated This class is deprecated and will removed in the next major release.
  * @param {olx.AttributionOptions} options Attribution options.
  * @struct
  * @api

src/ol/source/source.js

@@ -67,6 +67,7 @@ ol.inherits(ol.source.Source, ol.Object);
 
 /**
  * Turns the attributions option into an attributions function.
+ * @suppress {deprecated}
  * @param {ol.AttributionLike|undefined} attributionLike The attribution option.
  * @return {?ol.Attribution2} An attribution function (or null).
  */