vfio-ccw: Update documentation for csch/hsch

commit 127e6217 upstream.

We now support CLEAR SUBCHANNEL and HALT SUBCHANNEL
via ccw_cmd_region.

Fixes: d5afd5d1

 ("vfio-ccw: add handling for async channel instructions")
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Message-Id: <7d977612c3f3152ffb950d77ae11b4b25c1e20c4.1562854091.git.alifm@linux.ibm.com>
[CH: properly mark region as literal block]
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>