rdma: Document IB device renaming option

rdma: Document IB device renaming option

[leonro@server /]$ lspci |grep -i Ether
00:08.0 Ethernet controller: Red Hat, Inc. Virtio network device
00:09.0 Ethernet controller: Mellanox Technologies MT27700 Family [ConnectX-4]
[leonro@server /]$ sudo rdma dev
1: mlx5_0: node_type ca fw 3.8.9999 node_guid 5254:00c0:fe12:3455
sys_image_guid 5254:00c0:fe12:3455
[leonro@server /]$ sudo rdma dev set mlx5_0 name hfi1_0
[leonro@server /]$ sudo rdma dev
1: hfi1_0: node_type ca fw 3.8.9999 node_guid 5254:00c0:fe12:3455
sys_image_guid 5254:00c0:fe12:3455

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>