RDS: Documentation: Document AF_RDS, PF_RDS and SOL_RDS correctly.

AF_RDS, PF_RDS and SOL_RDS are available in header files,
and there is no need to get their values from /proc. Document
this correctly.

Fixes: 0c5f9b88

 ("RDS: Documentation")

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