[PATCH] s390: cio documentation update

Update documentation of the common I/O layer:
- Add MSS-specific example.
- Add more information on ccwgroup devices.
- Add channel path type attribute.
- Fix typo.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>