net: Document promote_secondaries

From 038a821667f62c496f2bbae27081b1b612122a97 Mon Sep 17 00:00:00 2001
From: Martin Schwenke <martin@meltin.net>
Date: Tue, 28 Jan 2014 15:16:49 +1100
Subject: [PATCH] net: Document promote_secondaries

This option was added a long time ago...

  commit 8f937c60
  Author: Harald Welte <laforge@gnumonks.org>
  Date:   Sun May 29 20:23:46 2005 -0700

    [IPV4]: Primary and secondary addresses

Signed-off-by: Martin Schwenke <martin@meltin.net>
Signed-off-by: David S. Miller <davem@davemloft.net>