xwayland: fix wlr_xwayland_surface_from_wlr_surface() docs
32daa43a 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.