[PATCH] PCI: document sysfs rom file interface

idr gently pointed out today that not only is the sysfs rom file
interface somewhat unintuitive (despite my efforts and initial
implementation), but it's also undocumented!  This patch to
Documentation/filesystems/sysfs-pci.txt corrects the latter problem; the
former is a userland ABI now though, so we're stuck with it for awhile
at least.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>