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>

Documentation/networking/ip-sysctl.txt

@@ -1088,6 +1088,12 @@ igmpv3_unsolicited_report_interval - INTEGER
 	IGMPv3 report retransmit will take place.
 	Default: 1000 (1 seconds)
 
+promote_secondaries - BOOLEAN
+	When a primary IP address is removed from this interface
+	promote a corresponding secondary IP address instead of
+	removing all the corresponding secondary IP addresses.
+
+
 tag - INTEGER
 	Allows you to write a number, which can be used as required.
 	Default value is 0.