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/notifications/basic-notification/main.js

@@ -20,6 +20,6 @@ function createWindow () {
   })
 }
 
-app.on('ready', () => {
+app.whenReady().then(() => {
   createWindow()
 })