docs: document cpu host-passthrough migration issue

Documents in formatdomain.html that when migrating a guest
defined with the host-passthrough CPU model from a machine that
is running on a newer CPU model than the destination machine's
CPU model, it is very likely that the guest will crash upon
arrival.
Signed-off-by: NJason J. Herne <jjherne@linux.vnet.ibm.com>

docs/formatdomain.html.in

@@ -1292,7 +1292,11 @@
           understand. Though the downside of this mode is that the guest
           environment cannot be reproduced on different hardware. Thus, if you
           hit any bugs, you are on your own. Further details of that CPU can
-          be changed using <code>feature</code> elements.</dd>
+          be changed using <code>feature</code> elements. Migration of a guest
+          using host-passthrough is dangerous if the source and destination
+          hosts are not identical in both hardware and configuration. If such
+          a migration is attempted then the guest may hang or crash upon
+          resuming execution on the destination host.</dd>
         </dl>
 
         In both <code>host-model</code> and <code>host-passthrough</code>