Add apply doc block. [ci skip]

lodash.js

@@ -371,6 +371,16 @@
     return set;
   }
 
+  /**
+   * A faster alternative to `Function#apply`, this function invokes `func`
+   * with the `this` binding of `thisArg` and the arguments of `args`.
+   *
+   * @private
+   * @param {Function} func The function to invoke.
+   * @param {*} thisArg The `this` binding of `func`.
+   * @param {...*} [args] The arguments to invoke `func` with.
+   * @returns {*} Returns the result of `func`.
+   */
   function apply(func, thisArg, args) {
     switch(args.length) {
       case 0: return func.call(thisArg);