[S390] Update documentation for dynamic subchannel mapping.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

Documentation/s390/driver-model.txt

@@ -18,11 +18,18 @@ devices/
 	   - 0.0.0002/
 	   - 0.1.0000/0.1.1234/
 	   ...
+	   - defunct/
 
 In this example, device 0815 is accessed via subchannel 0 in subchannel set 0,
 device 4711 via subchannel 1 in subchannel set 0, and subchannel 2 is a non-I/O
 subchannel. Device 1234 is accessed via subchannel 0 in subchannel set 1.
 
+The subchannel named 'defunct' does not represent any real subchannel on the
+system; it is a pseudo subchannel where disconnnected ccw devices are moved to
+if they are displaced by another ccw device becoming operational on their
+former subchannel. The ccw devices will be moved again to a proper subchannel
+if they become operational again on that subchannel.
+
 You should address a ccw device via its bus id (e.g. 0.0.4711); the device can
 be found under bus/ccw/devices/.