finalize/disconnect: add explicit comment about why we use an RTE barrier

Based on extensive discussions before/at the June 2014 developer's
meeting, put a lengthy comment explaining a second reason why we
''must'' use an RTE barrier during MPI_FINALIZE and
MPI_COMM_DISCONNECT (i.e., unreliable transports).  Slightly explain
more the original reason why we do this, too (BTLs can lie/buffer a
message without actually injecting it on the network).