Adds note for unexplained error messages

In the stable/icehouse branch for RHEL-based deployments,
it asked the user to start and enable dbus/messagebus. In
RHEL 7 this is not required as it's out of the box.

As Icehouse standardised on RHEL 6, we should keep it as
it is, but it's confusing if someone's deploying Icehouse
on RHEL 7 based platforms. This patch adds a note to advise.

References https://review.openstack.org/#/c/115753/

Change-Id: Ic16ff621b309aca270bca8bfe833553889a21c3d

doc/install-guide/section_nova-compute.xml

@@ -224,6 +224,10 @@ virt_type = qemu</programlisting>
 <prompt>#</prompt> <userinput>chkconfig libvirtd on</userinput>
 <prompt>#</prompt> <userinput>chkconfig dbus on</userinput>
 <prompt>#</prompt> <userinput>chkconfig openstack-nova-compute on</userinput></screen>
+        <note>
+        <para>On RHEL 7.x and derivatives, the dbus/messagebus commands listed above may
+          cause an error. These errors can be ignored because these services are now enabled by default.</para>
+        </note>
         </step>
       </stepalternatives>
       <screen os="opensuse;sles"><prompt>#</prompt> <userinput>service libvirtd start</userinput>