update docu on mpi_collective, bijective and write general collective
update docu on mpi_collective, bijective and write general collective

mwiesenberger/feltor,mwiesenberger/feltor,feltor-dev/feltor,mwiesenberger/feltor,feltor-dev/feltor,mwiesenberger/feltor,feltor-dev/feltor,feltor-dev/feltor