scsi: core: scmd_eh_abort_handler(): Add a comment

After the patch that introduced this function was posted on the
linux-scsi mailing list an explanation was posted why this patch is
correct. Since that explanation contains important information, add a
summary of it above the code that explanation applies to.  See also
http://www.spinics.net/lists/linux-scsi/msg106326.html.

References: e494f6a7

 ("[SCSI] improved eh timeout handler")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>