update zip docs.

lib/plugins/zip.js

@@ -16,7 +16,7 @@ var util = require('archiver-utils');
  * @param {Boolean} [options.forceZip64=false] Forces the archive to contain ZIP64 headers.
  * @param {Boolean} [options.store=false] Sets the compression method to STORE.
  * @param {Object} [options.zlib] Passed to [zlib]{@link https://nodejs.org/api/zlib.html#zlib_class_options}
- * @property {ZipStream} [engine]
+ * @property {ZipStream} engine The ZipStream instance that creates the zip.
  */
 var Zip = function(options) {
   if (!(this instanceof Zip)) {
@@ -84,8 +84,6 @@ Zip.prototype.unpipe = function() {
 
 module.exports = Zip;
 
-
-
 /**
  * @typedef {Object} ZipOptions
  * @global
@@ -112,7 +110,7 @@ module.exports = Zip;
  */
 
 /**
- * ZipStream
+ * @typedef {Object} ZipStream
  * @external ZipStream
  * @see {@link https://archiverjs.com/zip-stream/ZipStream.html}
  */