ENH: align MPI_Probe non-blocking handling with header description
- the header states that the commsType is non-blocking or not,
but the implementation actually checked for 'buffered' or not.
STYLE: fix some spacing and some documentation
- used Pstream::maxCommsSize (bytes) for the lower limit when sending.
This would have send more data on each iteration than expected based
on maxCommsSize and finish with a number of useless iterations.
Was generally not a serious bug since maxCommsSize (if used) was
likely still far away from the MPI limits and exchange() is primarily
harnessed by PstreamBuffers, which is sending character data
(ie, number of elements and number of bytes is identical).
