vfio: ccw: shorten kernel doc description for pfn_array_pin()

The kernel doc description for usage of the struct pfn_array in
pfn_array_pin() is unnecessary long. Let's shorten it by describing
the contents of the struct pfn_array fields at the struct's definition
instead.

Suggested-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Dong Jia Shi <bjsdjshi@linux.ibm.com>
Message-Id: <20180523025645.8978-2-bjsdjshi@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>