Documentation: RDS: updates for SO_RDS_TRANSPORT socket option

Update the documentation to describe the changes added by
commit 8ba38460 ("net/rds Add getsockopt support for SO_RDS_TRANSPORT")

Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>