[AF_IUCV]: Improve description of IUCV and AFIUCV configuration options.

Signed-off-by: Jennifer Hunt <jenhunt@us.ibm.com>
Signed-off-by: Ursula Braun <&gt;braunu@de.ibm.com>
Acked-by: Frank Pavlic <fpavlic@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>