[mpi3-coll] Additional edits 2

[Adam Moody]

Also against revision 3.

I'd like to make the language in the descriptions for the data movement 
collectives consistent across the different collectives.  The text skips 
around with several different flavors.  Personally, I like this template 
best:

    "MPI_Ixxx starts a nonblocking variant of MPI_Xxx.  The data 
movement after MPI_Ixxx completes is identical to the data movement 
after MPI_Xxx completes (see Section 5.Y)."

This would apply to bcast, gather, gatherv, scatter, scatterv, 
allgather, allgatherv, alltoall, alltoallv, and alltoallw.  We could add 
mention of inter- and intra-communicators here if folks would like to, 
but I like the simplicity of the above template myself -- leave all 
description up to the corresponding blocking section and provide a back 
reference.


Of couse, for the reductions: reduce, allreduce, reduce_scatter, scan, 
and exscan; we still need more discussion on what "delivers the same 
results" mean in the current description since the blocking and 
nonblocking operations may use different algorithms and we have the 
associativity issue.
-Adam