docs: Document the real behaviour of suspend-to-{mem,disk}

We get a question every now and then about why hibernation works when
suspend-to-disk is disabled and similar.  Let's hope that, by documenting the
obvious more blatantly, people will get more informed.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

docs/formatdomain.html.in

@@ -1664,7 +1664,11 @@
       <dd>These elements enable ('yes') or disable ('no') BIOS support
         for S3 (suspend-to-mem) and S4 (suspend-to-disk) ACPI sleep
         states. If nothing is specified, then the hypervisor will be
-        left with its default value.</dd>
+        left with its default value.<br/>
+        Note: This setting cannot prevent the guest OS from performing
+        a suspend as the guest OS itself can choose to circumvent the
+        unavailability of the sleep states (e.g. S4 by turning off
+        completely).</dd>
     </dl>
 
     <h3><a id="elementsFeatures">Hypervisor features</a></h3>