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>

drivers/s390/cio/vfio_ccw_cp.c

@@ -23,9 +23,13 @@
 #define CCWCHAIN_LEN_MAX	256
 
 struct pfn_array {
+	/* Starting guest physical I/O address. */
 	unsigned long		pa_iova;
+	/* Array that stores PFNs of the pages need to pin. */
 	unsigned long		*pa_iova_pfn;
+	/* Array that receives PFNs of the pages pinned. */
 	unsigned long		*pa_pfn;
+	/* Number of pages to pin/pinned from @pa_iova. */
 	int			pa_nr;
 };
 
@@ -53,14 +57,8 @@ struct ccwchain {
  * Attempt to pin user pages in memory.
  *
  * Usage of pfn_array:
- * @pa->pa_iova     starting guest physical I/O address. Assigned by caller.
- * @pa->pa_iova_pfn array that stores PFNs of the pages need to pin. Allocated
- *                  by caller.
- * @pa->pa_pfn      array that receives PFNs of the pages pinned. Allocated by
- *                  caller.
- * @pa->pa_nr       number of pages from @pa->pa_iova to pin. Assigned by
- *                  caller.
- *                  number of pages pinned. Assigned by callee.
+ * Any field in this structure should be initialized by caller.
+ * We expect @pa->pa_nr > 0, and its value will be assigned by callee.
  *
  * Returns:
  *   Number of pages pinned on success.