improves documentation of exchangers

src/xt/xt_redist_repeat.h

src/xt/xt_redist_single_array_base.h

src/xt_config.c

src/xt_config_internal.h

src/xt_exchanger_irecv_isend.c

src/xt_exchanger_irecv_isend.h

src/xt_exchanger_irecv_isend_packed.c

src/xt_exchanger_irecv_isend_packed.h

src/xt_exchanger_irecv_send.c

src/xt_exchanger_irecv_send.h

src/xt_exchanger_mix_isend_irecv.c

src/xt_exchanger_mix_isend_irecv.h

src/xt_exchanger_neigh_alltoall.c

src/xt_exchanger_neigh_alltoall.h

src/xt_exchanger_simple_base.c

src/xt_exchanger_simple_base.h