scsi: scsi_scan: Add kernel-doc for exported function

Add kernel-doc for scsi_add_device() since it is exported.  This allows it
to be part of the SCSI driver-api docbook.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20241212205217.597844-5-rdunlap@infradead.org


CC: James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
CC: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>