scsi: documentation: add scsi_mod.use_blk_mq to scsi-parameters

Kernel line argument scsi_mod.use_blk_mq is missing from file
Documentation/scsi/scsi-parameters.txt.  Add this option, providing
mention of config setting and format.

[mkp: clarified where to look]

Signed-off-by: John Pittman <jpittman@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>