Documentation: net-sysfs: describe missing statistics

Sync the ABI description with the interface statistics that are currently
available through sysfs.

CC: Jarod Wilson <jarod@redhat.com>
CC: Jonathan Corbet <corbet@lwn.net>
CC: linux-doc@vger.kernel.org
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>