[mpi3-coll] Comments on nonblocking collectives section

[Adam Moody]

For the most part, everyone's comments fit in fine with my reordering 
which I'll propose after Torsten integrates the comments from today.  I 
have just one conflict with one of Jesper's suggestions:

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>>p. 49, line 26:
>>"the performance of many systems..." -> 
>>"the performance of many applications can be improved by overlapping communication
>>and computation, and many systems enable this."
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>>
>fixed
>  
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I'd just simplify this to:
"As described in Section 3.7, performance on many systems can be 
improved by overlapping communication and computation."