Add send_arp_for_ha option to multihost-HA doc

fixes bug 1093000

The bug report explains well the issues that can be encountered
when using multihost HA flatDHCP, where ARP caches are not up to
 date. This patch adds the config option as a recommendation and
explains its purpose.

Change-Id: I0ca1e218c219f15f86f196531bb8a5fc342383dc

doc/src/docbkx/openstack-compute-admin/computenetworking.xml

@@ -2098,13 +2098,18 @@ iface eth1 inet dhcp                </programlisting></para>
                 </itemizedlist></para>
             <para>If the compute host is not an API endpoint, use the
                     <command>nova-api-metadata</command> service. The <filename>nova.conf</filename>
-                file should contain:<programlisting>multi_host=True</programlisting></para>
+                file should contain:<programlisting>multi_host=True
+send_arp_for_ha=true
+</programlisting>
+The <literal>send_arp_for_ha</literal> option facilitates sending of gratuituous arp messages
+to ensure the arp caches on compute hosts are up to date.</literal></para>
             <para>If a compute host is also an API endpoint, use the <command>nova-api</command>
                 service. Your <literal>enabled_apis</literal> option will need to contain
                     <literal>metadata</literal>, as well as additional options depending on the API
                 services. For example, if it supports compute requests, volume requests, and EC2
                 compatibility, the <filename>nova.conf</filename> file should contain:
                 <programlisting>multi_host=True
+send_arp_for_ha=true
 enabled_apis=ec2,osapi_compute,osapi_volume,metadata</programlisting></para>
 
             <para>The <literal>multi_host</literal> option must be in place for network creation and