message: Add some doc for nc_faces_orientation.
diff: @@ -152,6 +152,9 @@ protected:
  Array<FaceInfo> faces_info;
  Array<NCFaceInfo> nc_faces_info;
  + // This array stores the face orientation of non-conforming slave faces, the
  + // orientation of the slave face should be accessed with the ncface index
  + // given by Mesh::GetFaceInfos.
  Array<int> nc_faces_orientation;
  Table *el_to_edge;
  