scsi: Fix the kerneldoc for scsi_initialize_rq()

The kerneldoc comment for scsi_initialize_rq() neglected to document the
"rq" parameter, leading to this docs build warning:

  ./drivers/scsi/scsi_lib.c:1116: warning: No description found for parameter 'rq'

Document the parameter and make the build slightly quieter.

[mkp: used wording suggested by Bart]
Signed-off-by: NJonathan Corbet <corbet@lwn.net>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_lib.c

@@ -1108,6 +1108,7 @@ EXPORT_SYMBOL(scsi_init_io);
 
 /**
  * scsi_initialize_rq - initialize struct scsi_cmnd.req
+ * @rq: Request associated with the SCSI command to be initialized.
  *
  * Called from inside blk_get_request().
  */