[mpi3-coll] New nonblocking collective intro text

[Bronis R. de Supinski]

Torsten:

Re:
> collective operations on overlapping communicators that would lead to
> "that" -> "which"

Huh? The rev3 PDF still has that. I don't have a strong
opinion over whether the clause is required but I think
you are right that it is not. So, you should change it
to ", which" (make sure you include the comma).

Related to this, currently on p49, lines 45-47, you have:

> The start call initiates the operation which indicates that
> the system may start to copy data out of the send buffer and
> into the receive buffer.

A comma is needed before the "which":

 The start call initiates the operation, which indicates that
 the system may start to copy data out of the send buffer and
 into the receive buffer.

Otherwise, the current version looks good to me.

Bronis