trivial: doc: hpfall: accept disk device to unload as argument

Allows users who use an IDE driver for their disk to use hpfall without
having to modify the source. By default /dev/sda is used.
Suggested by Christian Thaeter in http://lkml.org/lkml/2009/3/25/505.

While we're add it, improve error message if opening /dev/freefall fails.

Signed-off-by: Frans Pop <elendil@planet.nl>
Cc: Christian Thaeter <ct@pipapo.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>