docs: clarify before `ready` usage (#41849)

docs/api/app.md

@@ -41,6 +41,10 @@ that was used to open the application, if it was launched from Notification Cent
 You can also call `app.isReady()` to check if this event has already fired and `app.whenReady()`
 to get a Promise that is fulfilled when Electron is initialized.
 
+**Note**: The `ready` event is only fired after the main process has finished running the first
+tick of the event loop. If an Electron API needs to be called before the `ready` event, ensure
+that it is called synchronously in the top-level context of the main process.
+
 ### Event: 'window-all-closed'
 
 Emitted when all windows have been closed.