clarified applyEach arguments, fixes #1278

lib/applyEach.js

@@ -4,23 +4,25 @@ import map from './map';
 /**
  * Applies the provided arguments to each function in the array, calling
  * `callback` after all functions have completed. If you only provide the first
- * argument, then it will return a function which lets you pass in the
- * arguments as if it were a single function call.
+ * argument, `fns`, then it will return a function which lets you pass in the
+ * arguments as if it were a single function call. If more arguments are
+ * provided, `callback` is required while `args` is still optional.
  *
  * @name applyEach
  * @static
  * @memberOf module:ControlFlow
  * @method
  * @category Control Flow
- * @param {Array|Iterable|Object} fns - A collection of asynchronous functions to all
- * call with the same arguments
+ * @param {Array|Iterable|Object} fns - A collection of asynchronous functions
+ * to all call with the same arguments
  * @param {...*} [args] - any number of separate arguments to pass to the
  * function.
  * @param {Function} [callback] - the final argument should be the callback,
  * called when all functions have completed processing.
- * @returns {Function} - If only the first argument is provided, it will return
- * a function which lets you pass in the arguments as if it were a single
- * function call.
+ * @returns {Function} - If only the first argument, `fns`, is provided, it will
+ * return a function which lets you pass in the arguments as if it were a single
+ * function call. The signature is `(..args, callback)`. If invoked with any
+ * arguments, `callback` is required.
  * @example
  *
  * async.applyEach([enableSearch, updateSchema], 'bucket', callback);