message: update review
  documentation
diff: @@ -442,6 +442,7 @@ public:
  * original MPI_Comm object.
  *
  * @param comm The input MPI_Comm object.
  + * @param force_host_buffer If set to true, always communicates through host memory
  */
  communicator(const MPI_Comm& comm, bool force_host_buffer = false)
  : comm_(), force_host_buffer_(force_host_buffer)
  