Add comment about the usage in the friend class.

include/DeviceModule.h

@@ -267,7 +267,9 @@ namespace ChimeraTK {
     boost::shared_mutex recoverySharedMutex;
 
     friend class Application;
+    // Access to virtualiseFromCatalog() is needed by ServerHistory
     friend struct history::ServerHistory;
+    // Access to virtualiseFromCatalog() is needed by MicroDAQ
     template<typename TRIGGERTYPE>
     friend class MicroDAQ;
     friend struct detail::DeviceModuleProxy;