ide: note that IDE generic may prevent other drivers from attaching

Enabling IDE generic may prevent ATA controllers located on legacy
ports from being attached to more proper driver or can prevent other
controllers which share the IRQ from working.  Note it in the help
message.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: xerces8 <xerces8@butn.net>
Cc: Jeff Garzik <jgarzik@pobox.com>
Cc: stein@hermes.si
[bart: s/will grab/may grab/ since Borislav has fixed PCI-case for .28]
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>