[update] scsi_mid_low_api.txt

[update] scsi_mid_low_api.txt

Here is another version of the scsi_mid_low_api.txt
document. It adds descriptions of functions supplied
by the mid level for a LLD (lower level driver) to call.
It is also tries to describe the two, alternate
registration scenarios. How do the terms:
"hotplug initialization model" and "passive
initialization model" sound?