docs: Document configuration of HPT resizing

Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

docs/formatdomain.html.in

@@ -1771,7 +1771,7 @@
   <pvspinlock state='on'/>
   <gic version='2'/>
   <ioapic driver='qemu'/>
-
+  <hpt resizing='required'/>
 </features>
 ...</pre>
 
@@ -1940,6 +1940,17 @@
           which is also known as a split I/O APIC mode.
           <span class="since">Since 3.4.0</span> (QEMU/KVM only)
       </dd>
+      <dt><code>hpt</code></dt>
+      <dd>Configure the HPT (Hash Page Table) of a pSeries guest. Possible
+          values for the <code>resizing</code> attribute are
+          <code>enabled</code>, which causes HPT resizing to be enabled if
+          both the guest and the host support it; <code>disabled</code>, which
+          causes HPT resizing to be disabled regardless of guest and host
+          support; and <code>required</code>, which prevents the guest from
+          starting unless both the guest and the host support HPT resizing. If
+          the attribute is not defined, the hypervisor default will be used.
+          <span class="since">Since 3.10.0</span> (QEMU/KVM only)
+      </dd>
     </dl>
 
     <h3><a id="elementsTime">Time keeping</a></h3>