refactor: prefer using app.whenReady() (#21972)

* docs: add references to app.whenReady() in isReady

* refactor: prefer app.whenReady()

In the docs, specs, and lib, replace instances of `app.once('ready')`
(seen occasionally) and `app.on('ready')` (extremely common) with
`app.whenReady()`.

It's better to encourage users to use whenReady():
1. it handles the edge case of registering for 'ready' after it's fired
2. it avoids the minor wart of leaving an active listener alive for
an event that wll never fire again

docs/fiddles/native-ui/tray/main.js

@@ -34,7 +34,7 @@ function createWindow () {
 // This method will be called when Electron has finished
 // initialization and is ready to create browser windows.
 // Some APIs can only be used after this event occurs.
-app.on('ready', createWindow)
+app.whenReady().then(createWindow)
 
 // Quit when all windows are closed.
 app.on('window-all-closed', function () {