clarified spec

dlib/gui_widgets/widgets_abstract.h

@@ -2725,8 +2725,8 @@ namespace dlib
             requires
                 - event_handler is a valid pointer to a member function in T 
             ensures
-                - the event_handler function is called on object when the user adds
-                  or removes an overlay rectangle.
+                - the event_handler function is called on object when the user adds,
+                  removes, or modifies an overlay rectangle.
                 - any previous calls to this function are overridden by this new call.  
                   (i.e. you can only have one event handler associated with this 
                   event at a time)