update the BLK_DEV_HD help text

Many people will see this option the first time now that it is in
drivers/block/

Make it clear that virtually noone needs it.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Cc: rmk@arm.linux.org.uk
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>