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: NBart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: NHannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_error.c

@@ -117,6 +117,12 @@ static int scsi_host_eh_past_deadline(struct Scsi_Host *shost)
 /**
  * scmd_eh_abort_handler - Handle command aborts
  * @work:	command to be aborted.
+ *
+ * Note: this function must be called only for a command that has timed out.
+ * Because the block layer marks a request as complete before it calls
+ * scsi_times_out(), a .scsi_done() call from the LLD for a command that has
+ * timed out do not have any effect. Hence it is safe to call
+ * scsi_finish_command() from this function.
  */
 void
 scmd_eh_abort_handler(struct work_struct *work)