docs: Enhance memoryBacking/locked documentation

Mention the need to set memtune/hard_limit.

https://bugzilla.redhat.com/show_bug.cgi?id=1035954

docs/formatdomain.html.in

@@ -638,7 +638,10 @@
       <dt><code>locked</code></dt>
       <dd>When set and supported by the hypervisor, memory pages belonging
         to the domain will be locked in host's memory and the host will not
-        be allowed to swap them out.
+        be allowed to swap them out. For QEMU/KVM this requires
+        <code>hard_limit</code> <a href="#elementsMemoryTuning">memory tunning</a>
+        element to be used and set to the maximum memory configured for the
+        domain plus any memory consumed by the QEMU process itself.
         <span class="since">Since 1.0.6</span></dd>
     </dl>