docs: revised the macos dock menu feature page (electron#25985)

* docs: revised the macos dock menu feature page

* docs: added a cross-link to api, fixed mentions in the macos feature page

docs/tutorial/macos-dock.md

@@ -1,10 +1,11 @@
 # macOS Dock
 
+## Overview
+
 Electron has APIs to configure the app's icon in the macOS Dock. A macOS-only
-API exists to create a custom dock menu, but
-Electron also uses the app's dock icon to implement cross-platform features
-like [recent documents][recent-documents] and
-[application progress][progress-bar].
+API exists to create a custom dock menu, but Electron also uses the app dock
+icon as the entry point for cross-platform features like
+[recent documents][recent-documents] and [application progress][progress-bar].
 
 The custom dock is commonly used to add shortcuts to tasks the user wouldn't
 want to open the whole app window for.
@@ -13,8 +14,15 @@ __Dock menu of Terminal.app:__
 
 ![Dock Menu][dock-menu-image]
 
-To set your custom dock menu, you can use the `app.dock.setMenu` API, which is
-only available on macOS:
+To set your custom dock menu, you need to use the
+[`app.dock.setMenu`](../api/dock.md#docksetmenumenu-macos) API,
+which is only available on macOS.
+
+## Example
+
+Starting with a working application from the
+ [Quick Start Guide](quick-start.md), update the `main.js` file with the
+ following lines:
 
 ```javascript
 const { app, Menu } = require('electron')
@@ -33,9 +41,16 @@ const dockMenu = Menu.buildFromTemplate([
   { label: 'New Command...' }
 ])
 
-app.dock.setMenu(dockMenu)
+app.whenReady().then(() => {
+  app.dock.setMenu(dockMenu)
+})
 ```
 
+After launching the Electron application, right click the application icon.
+You should see the custom menu you just defined:
+
+![macOS dock menu](../images/macos-dock-menu.png)
+
 [dock-menu-image]: https://cloud.githubusercontent.com/assets/639601/5069962/6032658a-6e9c-11e4-9953-aa84006bdfff.png
 [recent-documents]: ./recent-documents.md
 [progress-bar]: ./progress-bar.md