Documentation/ABI/rnbd-clt: session name is appended to the device path

When mapping a device,
/sys/devices/virtual/rnbd-client/ctl/devices/<device_id> was created.
But we found out that it had a problem when mapping the same file
on different servers. So we append the session name after the
device_id as below.
/sys/devices/virtual/rnbd-client/ctl/devices/<device_id>@<session_name>

Signed-off-by: Gioh Kim <gi-oh.kim@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>