[SCSI] scsi_mid_low_api.txt recommend resid usage

As discussed in a thread on this list titled:
    "RFC: short reads on block devices"
this patch adds recommendations for LLDs to set resid
when there might be uncertainty about how much data
has been returned by a device.

This patch inline and attached] is against scsi-misc-2.6.git

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>