[Mpi3-rma] MPI_NO_OP and origin buffer

Hi All, It looks like we forgot to specify that for MPI_NO_OP in the new accumulate ops, the origin buffer is ignored (i.e. it's acceptable to pass NULL). We can add this text on page 441, line 30: When MPI_NO_OP is specified, the origin buffer is ignored. We already have such a use case in the linked list example. ~Jim.