[mpi3-coll] More on in-place

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