docs: document when BrowserWindow and BrowserView can be used (#33696)

We already document such info for other APIs, like the 'screen' API in
f711fe6b57/docs/api/screen.md (L7-L8).
So we should do the same thing for these ones too.

Signed-off-by: Darshan Sen <raisinten@gmail.com>

docs/api/browser-view.md

@@ -11,6 +11,9 @@ relative to its owning window. It is meant to be an alternative to the
 
 Process: [Main](../glossary.md#main-process)
 
+This module cannot be used until the `ready` event of the `app`
+module is emitted.
+
 ### Example
 
 ```javascript

docs/api/browser-window.md

@@ -4,6 +4,9 @@
 
 Process: [Main](../glossary.md#main-process)
 
+This module cannot be used until the `ready` event of the `app`
+module is emitted.
+
 ```javascript
 // In the main process.
 const { BrowserWindow } = require('electron')