Add comments for add/clear slots.

PropertyVisPlugin.hh

@@ -149,8 +149,16 @@ private slots:
 
 
 public slots:
+  /// Visualise a property on object _id with the given name.
+  /// Note that for multiple properties with the same name (i.e., different
+  ///  data types or entity types), there is no guarantee which one is used.
   void slotVisualizeProperty( int _id, const QString& _propname );
+
+  /// Remove visualisation of a property on object _id with the given name.
+  /// Note that for multiple properties with the same name (i.e., different
+  ///  data types or entity types), there is no guarantee which one is used.
   void slotClear( int _id, const QString& _propname );
+
   QScriptValue getPropertyVisualizer( int _id, const QString& _propname );