thorium_transport: fix documentation of mock_transport

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_transport: remove outdated constant

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: don't use a aloop in mpi1_pt2pt_transport_test

The function thorium_transport_receive is already called in a
loop in node.c

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
documentation: improve transport interface documentation

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>