Documentation/ABI: document /sys/class/net/*/dev_port

The sysfs field was introduced 4 years ago along with fixes to various
drivers that erroneously used `dev_id' for that purpose, but it was not
properly documented anywhere.
See commit v3.14-rc3-739-g3f85944fe207.

Signed-off-by: Arseny Maslennikov <ar@cs.msu.ru>
Signed-off-by: Doug Ledford <dledford@redhat.com>