Edit "quit()" method description in `SceneTree`

Updated the `SceneTree.quit()` method to include a note that on iOS this method won't work as apps are expected to be closed via the Home button, not programmatically.

(cherry picked from commit c63af17a31b562f4355de6d715d00a3646e552c7)

doc/classes/SceneTree.xml

@@ -196,6 +196,7 @@
 			</argument>
 			<description>
 				Quits the application at the end of the current iteration. A process [code]exit_code[/code] can optionally be passed as an argument. If this argument is [code]0[/code] or greater, it will override the [member OS.exit_code] defined before quitting the application.
+				[b]Note:[/b] On iOS this method doesn't work. Instead, as recommended by the iOS Human Interface Guidelines, the user is expected to close apps via the Home button.
 			</description>
 		</method>
 		<method name="reload_current_scene">