Window documentation should mention the Embed Subwindows project setting

**Your Godot version:**
4.1

**Issue description:**
I couldn't figure out why the Window nodes in my scene were embedded in the viewport instead of being native windows. The documentation makes many mentions of native windows and even the Viewport.gui_embed_subwindows property, but nothing mentioned that I needed to change this project setting.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_window.html