docs: formatdomain: Clarify CPU feature policy option "require"

Using the 'require' value for the 'policy' attribute indicates
that the guest will have the feature so the host CPU does not
need to support it if the hypervisor can emulate it.

E.g. 'x2apic' is emulated by QEMU even if the host does not support it:

    <feature policy='require' name='x2apic'/>
Signed-off-by: NKashyap Chamarthy <kchamart@redhat.com>

docs/formatdomain.html.in

@@ -1531,8 +1531,8 @@
           <dd>The virtual CPU will claim the feature is supported regardless
             of it being supported by host CPU.</dd>
           <dt><code>require</code></dt>
-          <dd>Guest creation will fail unless the feature is supported by host
-            CPU.</dd>
+          <dd>Guest creation will fail unless the feature is supported by the
+            host CPU or the hypervisor is able to emulate it.</dd>
           <dt><code>optional</code></dt>
           <dd>The feature will be supported by virtual CPU if and only if it
             is supported by host CPU.</dd>