ide: update warm-plug HOWTO

Reported-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>

Documentation/ide/warm-plug-howto.txt

@@ -11,3 +11,8 @@ unplug old device(s) and plug new device(s)
 # echo -n "1" > /sys/class/ide_port/idex/scan
 
 done
+
+NOTE: please make sure that partitions are unmounted and that there are
+no other active references to devices before doing "delete_devices" step,
+also do not attempt "scan" step on devices currently in use -- otherwise
+results may be unpredictable and lead to data loss if you're unlucky