docs: Add Fibre Channel NPIV supported option for volume lun config

"Further" clarification (and testing) shows that using a SCSI Fibre
Channel NPIV device/lun from a storage pool as a <disk type='volume'
device'lun'> will work. So just add that to the allowable options

Related to: https://bugzilla.redhat.com/show_bug.cgi?id=1230179

docs/formatdomain.html.in

@@ -1973,7 +1973,9 @@
             valid when the <code>type</code> is "block" or "network" for
             <code>protocol='iscsi'</code> or when the <code>type</code>
             is "volume" when using an iSCSI source <code>pool</code>
-            for <code>mode</code> "host".
+            for <code>mode</code> "host" or as an
+            <a href="http://wiki.libvirt.org/page/NPIV_in_libvirt">NPIV</a>
+            virtual Host Bus Adapter (vHBA) using a Fibre Channel storage pool.
             Configured in this manner, the LUN behaves identically to "disk",
             except that generic SCSI commands from the guest are accepted
             and passed through to the physical device. Also note that