Add links to the backend api docs.
### XDG_RUNTIME_DIR
Used to locate the Wayland socket to use.
+
+## Wayland-specific APIs
+
+See the [documentation](https://docs.gtk.org/gdk4-wayland/) for
+Wayland-specific GDK APIs.
for each monitor from the display system. This environment variable
allows to override that.
+## X11-specific APIs
+
+See the [documentation](https://docs.gtk.org/gdk4-x11/) for
+X11-specific GDK APIs.
+
## Understanding the X11 architecture
People coming from a Windows or MacOS background often find certain