docs: Clarification for when allowed to use 'lun' for "volume"

While re-reading what I wrote for commit id '785a8940', I realized
I needed to clarify that being able to present as a 'lun', the mode
property for the pool source element needed to be "host" (or empty)
and not "direct".

It was described correctly later in the mode host description, but
this just ensures it's not missed here as well.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

docs/formatdomain.html.in

@@ -1972,7 +1972,8 @@
             Using "lun" (<span class="since">since 0.9.10</span>) is only
             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>.
+            is "volume" when using an iSCSI source <code>pool</code>
+            for <code>mode</code> "host".
             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