docs: Clarify semantics of model/@usable attribute in dom caps

The documentation could confuse people to expect that CPU models with
usable='no' attribute are not usable at all on the current host. But
they cannot be only used without explicitly disabling some features.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>

docs/formatdomaincaps.html.in

@@ -249,9 +249,11 @@
         The <code>mode</code> element contains a list of supported CPU
         models, each described by a dedicated <code>model</code> element.
         The <code>usable</code> attribute specifies whether the model can
-        be used on the host. A special value <code>unknown</code> indicates
-        libvirt does not have enough information to provide the usability
-        data.
+        be used directly on the host. When usable='no' the corresponding model
+        cannot be used without disabling some features that the CPU of such
+        model is expected to have. A special value <code>unknown</code>
+        indicates libvirt does not have enough information to provide the
+        usability data.
       </dd>
     </dl>