More precise {function} jsdoc definition

src/ol/collection.js

@@ -112,11 +112,11 @@ ol.Collection.prototype.extend = function(arr) {
 
 /**
  * Iterate over each element, calling the provided callback.
- * @param {Function} f The function to call for every element. This function
- *     takes 3 arguments (the element, the index and the array). The return
- *     value is ignored.
- * @param {Object=} opt_obj The object to be used as the value of 'this'
- *     within f.
+ * @param {function(this: S, T, number, Array.<T>): ?} f The function to call
+ *     for every element. This function takes 3 arguments (the element, the
+ *     index and the array). The return value is ignored.
+ * @param {S=} opt_obj The object to be used as the value of 'this' within f.
+ * @template T,S
  * @todo stability experimental
  */
 ol.Collection.prototype.forEach = function(f, opt_obj) {