Improve confusing comment in wxMSW wxExecute() implementation

The comment was difficult to understand because of the missing "in"
preposition, so add it and extend the comment further for clarity.

See https://github.com/wxWidgets/wxWidgets/pull/369

src/msw/utilsexc.cpp

@@ -921,7 +921,9 @@ long wxExecute(const wxString& cmd, int flags, wxProcess *handler,
     if ( flags & wxEXEC_SYNC )
     {
         // handler may be !NULL for capturing program output, but we don't use
-        // it wxExecuteData struct in this case
+        // it in wxExecuteData struct in this case because it's only needed
+        // there for calling OnTerminate() on it and we don't do this when
+        // executing synchronously
         data->handler = NULL;
     }
     else