staging/tearing-control: clarify what happens after wl_surface destruction

There's no protocol error for making requests on the object after the wl_surface
has been destroyed, so the object has to become inert in that case.

Signed-off-by: Xaver Hugl <xaver.hugl@kde.org>

staging/tearing-control/tearing-control-v1.xml

@@ -75,6 +75,9 @@
       presentation with tearing.
       The default presentation hint is vsync. See presentation_hint for more
       details.
+
+      If the associated wl_surface is destroyed, this object becomes inert and
+      should be destroyed.
     </description>
 
     <enum name="presentation_hint">