docs(api.md): clarify docs about waitForNavigation (#2788)

lib/Page.js

@@ -616,7 +616,7 @@ class Page extends EventEmitter {
 
   /**
    * @param {!Object=} options
-   * @return {!Promise<!Puppeteer.Response>}
+   * @return {!Promise<?Puppeteer.Response>}
    */
   async waitForNavigation(options = {}) {
     const mainFrame = this._frameManager.mainFrame();