scsi: Clarify SCSI core module parameter documentation

Ever since it became possible to compile the SCSI core code as a module,
the documentation describing the module parameters has been incorrect.
Update the documentation to add a "scsi_mod." prefix to the relevant
options.

Reported-by: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>