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