message: Add description of ApplicationManagerImpl::AddMockApplication()
  Reflecting review comments.
diff: @@ -1559,8 +1559,12 @@ class ApplicationManagerImpl
  volatile bool is_stopping_;
  #ifdef BUILD_TESTS
  - // Methods for test usage
  public:
  + /**
  + * @brief register a mock application without going through the formal
  + * registration process. Only for unit testing.
  + * @param mock_app the mock app to be registered
  + */
  void AddMockApplication(ApplicationSharedPtr mock_app);
  #endif
  