[mpi3-coll] More on in-place

[Adam Moody]

I think we need another statement about the in-place option in the text 
regarding buffer access.  Something along the lines of:

When using the "in-place" option, message buffers may function as both 
send and receive buffers.  Such buffers should not be modified or 
accessed until the operation completes.

This is somewhat unique to NBC.  However, this statement is necessary 
for blocking collectives in threaded environments, as well.  Did the 
"Send buffer access" ticket resolve this?  Either way, it might not hurt 
to reiterate this point in the NBC intro, since it is more likely to 
come up there.
-Adam