add function documentation
implementing v-collectives using nbhd_alltoallw

neighborhood_alltoallw is special in that it has MPI_Aint displacements
and thus can be used to implement large-count v-collectives properly.

this commit only adds the implementation of large-count alltoallw in
terms of neighborhood_alltoallw.  then, at least for starters, we can
implement all of the other v-collectives in terms of alltoallw to save
time.  well, except for the ones that need a positive root argument...
we need rank/size in all cases
remove comment about specialization because we implemented it