Add 'short help text' to ATA_ACPI so it's [de]selectable.

ATA_ACPI isn't selectable right now because it lacks 'short help
text'.  This means it's always enabled and always enables ACPI_DOCK.
Add text so it's now [de]selectable.

cc: Jeff Garzik <jeff@garzik.org>
cc: Tejun Heo <htejun@gmail.com>
cc: Len Brown <len.brown@intel.com>
Signed-off-by: Chris Wedgwood <cw@f00f.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>