fix doxygen and tweak API

src/core/model/simulator.cc

@@ -190,11 +190,12 @@ Simulator::Stop()
     GetImpl()->Stop();
 }
 
-void
+EventId
 Simulator::Stop(const Time& delay)
 {
     NS_LOG_FUNCTION(delay);
     m_stopEvent = GetImpl()->Stop(delay);
+    return m_stopEvent;
 }
 
 EventId

src/core/model/simulator.h

@@ -166,12 +166,14 @@ class Simulator
      * or equal to the stop time.  The stop time is relative to the
      * current simulation time.
      * @param [in] delay The stop time, relative to the current time.
+     * @return The stop EventId.
      */
-    static void Stop(const Time& delay);
+    static EventId Stop(const Time& delay);
 
     /**
      * Returns the Stop Event, or an invalid event if the simulation
      * does not have a scheduled stop time.
+     * @return The stop EventId.
      */
     static EventId GetStopEvent();