docs: Refer to backend APIs

Add links to the backend api docs.

docs/reference/gtk/wayland.md

@@ -18,3 +18,8 @@ or wayland-1.
 ### 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.

docs/reference/gtk/x11.md

@@ -25,6 +25,11 @@ high-dpi displays. Normally, GDK will pick up a suitable scale factor
 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