docs: BrowserWindow extension APIs are deprecated in Electron 9 (#26781)

Co-authored-by: Milan Burda <milan.burda@gmail.com>

docs/breaking-changes.md

@@ -186,6 +186,45 @@ you should plan to update your native modules to be context aware.
 
 For more detailed information see [#18397](https://github.com/electron/electron/issues/18397).
 
+### Deprecated: `BrowserWindow` extension APIs
+
+The following extension APIs have been deprecated:
+* `BrowserWindow.addExtension(path)`
+* `BrowserWindow.addDevToolsExtension(path)`
+* `BrowserWindow.removeExtension(name)`
+* `BrowserWindow.removeDevToolsExtension(name)`
+* `BrowserWindow.getExtensions()`
+* `BrowserWindow.getDevToolsExtensions()`
+
+Use the session APIs instead:
+* `ses.loadExtension(path)`
+* `ses.removeExtension(extension_id)`
+* `ses.getAllExtensions()`
+
+```js
+// Deprecated in Electron 9
+BrowserWindow.addExtension(path)
+BrowserWindow.addDevToolsExtension(path)
+// Replace with
+session.defaultSession.loadExtension(path)
+```
+
+```js
+// Deprecated in Electron 9
+BrowserWindow.removeExtension(name)
+BrowserWindow.removeDevToolsExtension(name)
+// Replace with
+session.defaultSession.removeExtension(extension_id)
+```
+
+```js
+// Deprecated in Electron 9
+BrowserWindow.getExtensions()
+BrowserWindow.getDevToolsExtensions()
+// Replace with
+session.defaultSession.getAllExtensions()
+```
+
 ### Removed: `<webview>.getWebContents()`
 
 This API, which was deprecated in Electron 8.0, is now removed.