message: Add comment to Group ctor
diff: @@ -1176,6 +1176,9 @@ private:
  /*!
  * \brief Private ctor that creates a Group with given name
  * in the given DataStore.
  + *
  + * attachGroup must be called on a newly created Group to insert it
  + * into the hierarchy. The root group is an exception to this rule.
  */
  Group(const std::string& name, DataStore * datastore);
  