xwayland: fix wlr_xwayland_surface_from_wlr_surface() docs

32daa43a45 has removed the asymmetry in
the relationship of a wlr_surface and an unmapped wlr_xwayland_surface,
when wlr_surface.role_data wasn't NULL but wlr_xwayland_surface.surface
was. However, this also means that
wlr_xwayland_surface_from_wlr_surface() now returns NULL if the
wlr_surface is unmapped. Fix the documentation to reflect this.

include/wlr/xwayland/xwayland.h

@@ -232,7 +232,7 @@ bool wlr_surface_is_xwayland_surface(struct wlr_surface *surface);
  * Get a struct wlr_xwayland_surface from a struct wlr_surface.
  * Asserts that the surface has the xwayland surface role.
  * May return NULL even if the surface has the xwayland surface role if the
- * corresponding xwayland surface has been destroyed.
+ * corresponding xwayland surface has been unmapped or destroyed.
  */
 struct wlr_xwayland_surface *wlr_xwayland_surface_from_wlr_surface(
 	struct wlr_surface *surface);