docs: Annotate StageManager

The ClutterStageManager instance structure is private, so we need a
"floating" annotation.

clutter/clutter-stage-manager.h

@@ -43,6 +43,14 @@ G_BEGIN_DECLS
 typedef struct _ClutterStageManager             ClutterStageManager;
 typedef struct _ClutterStageManagerClass        ClutterStageManagerClass;
 
+/**
+ * ClutterStageManager:
+ *
+ * The #ClutterStageManager structure is private.
+ *
+ * Since: 1.0
+ */
+
 /**
  * ClutterStageManagerClass;
  * @stage_added: class handler for the #ClutterStageManager::stage-added