Added doxygen comment for the Trim_2 functor (which resolved a warning)

Arrangement_on_surface_2/doc/Arrangement_on_surface_2/CGAL/Arr_segment_traits_2.h

@@ -111,6 +111,7 @@ public:
   //! The curve type.
   typedef X_monotone_curve_2                    Curve_2;
 
+  //! A functor that trims curves.
   Class Trim_2 {
   public:
     //! \name Creation
@@ -122,7 +123,6 @@ public:
     X_monotone_curve_2(const X_monotone_curve_2& xcv,
                        const Point_2& src, const Point_2& tgt) const;
 
-    //! @}
   } /* end Arr_segment_traits_2::Trim_2 */
 
 }; /* end Arr_segment_traits_2 */