[stage] Add a warning for :offscreen usage

The Stage:offscreen property hasn't been tested for ages, and it
should really just use a FBO, not indirect rendering on a X Pixmap
only on X11. There are better ways anyway to get the current
contents of ClutterStage as a buffer anyway.

We might remove it at any later date, or actually make it work
properly.

clutter/clutter-stage.c

@@ -797,6 +797,10 @@ clutter_stage_class_init (ClutterStageClass *klass)
    * ClutterStage:offscreen:
    *
    * Whether the stage should be rendered in an offscreen buffer.
+   *
+   * <warning><para>Not every backend supports redirecting the
+   * stage to an offscreen buffer. This property might not work
+   * and it might be deprecated at any later date.</para></warning>
    */
   pspec = g_param_spec_boolean ("offscreen",
                                 "Offscreen",