IB/rxe: Update documentation link

All Soft-RoCE (rxe) is handled now in rdma-core user space library,
so the documentation. The patch below updates the documentation
link to that new location.

Reported-by: Josh Beavers <josh.beavers@gmail.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>