document promote_secondaries

Write something about the tweak to enable promoting secondary addresses
instead of deleting them together with the primary address as discussed
in this thread on the netdev mailing list:
http://www.spinics.net/lists/netdev/msg52294.html

The claim that this is supported since 2.6.15 is based on looking at
changes to net/ipv4/devinet.c in the linux-2.6 git tree:

Commit to add support:
2005-05-30      Harald Welte
8f937c6099858eee15fae14009dcbd05177fa91d

Commit to fix bug:
2005-11-22      Jamal Hadi Salim
0ff60a45678e67b2547256a636fd00c1667ce4fa

Version 2.6.15 was released 2006-01-03 and seems to include the code from
both the above commits.

doc/ip-cref.tex

@@ -751,6 +751,11 @@ An IP address becomes secondary if another address with the same
 prefix bits already exists. The first address is primary.
 It is the leader of the group of all secondary addresses. When the leader
 is deleted, all secondaries are purged too.
+There is a tweak in \verb|/proc/sys/net/ipv4/conf/<dev>/promote_secondaries|
+which activate secondaries promotion when a primary is deleted.
+To permanently enable this feature on all devices add
+\verb|net.ipv4.conf.all.promote_secondaries=1| to \verb|/etc/sysctl.conf|.
+This tweak is available in linux 2.6.15 and later.
 
 
 \item \verb|dynamic|