[SCSI] boot options: correct option name and tell where to find docs for it

Minor corrections and additions to 'scsi_logging_level', as pointed out
by Chuck Ebbert.
Also point out the IBM S390-tools 'scsi_logging_level' script.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>