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

Documentation/networking/rds.txt

@@ -85,7 +85,8 @@ Socket Interface
 
   bind(fd, &sockaddr_in, ...)
         This binds the socket to a local IP address and port, and a
-        transport.
+        transport, if one has not already been selected via the
+	SO_RDS_TRANSPORT socket option
 
   sendmsg(fd, ...)
         Sends a message to the indicated recipient. The kernel will
@@ -146,6 +147,20 @@ Socket Interface
         operation. In this case, it would use RDS_CANCEL_SENT_TO to
         nuke any pending messages.
 
+  setsockopt(fd, SOL_RDS, SO_RDS_TRANSPORT, (int *)&transport ..)
+  getsockopt(fd, SOL_RDS, SO_RDS_TRANSPORT, (int *)&transport ..)
+	Set or read an integer defining  the underlying
+	encapsulating transport to be used for RDS packets on the
+	socket. When setting the option, integer argument may be
+	one of RDS_TRANS_TCP or RDS_TRANS_IB. When retrieving the
+	value, RDS_TRANS_NONE will be returned on an unbound socket.
+	This socket option may only be set exactly once on the socket,
+	prior to binding it via the bind(2) system call. Attempts to
+	set SO_RDS_TRANSPORT on a socket for which the transport has
+	been previously attached explicitly (by SO_RDS_TRANSPORT) or
+	implicitly (via bind(2)) will return an error of EOPNOTSUPP.
+	An attempt to set SO_RDS_TRANSPPORT to RDS_TRANS_NONE will
+	always return EINVAL.
 
 RDMA for RDS
 ============