PCI: add some sysfs ABI docs

Add sysfs ABI docs for driver entries bind, unbind and new_id.  These
entries are pretty old, from 2.6.0 onwards AFAIK, so this documents
current behaviour.

Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Jesse Barnes <jbarnes@hobbes.lan>