docs: add a note that attr 'managed' is only used by PCI devices

Our documentation isn't 100% clear about hostdev 'managed' attribute usage,
because it only makes sense to use it with PCI devices, yet we format
this attribute to all hostdev devices. By adding a note into the docs,
we can possibly avoid confusion from customer's side and also avoid a solution
using ternary logic.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1155887
(cherry picked from commit ccfe9e48)

docs/formatdomain.html.in

@@ -3110,6 +3110,12 @@
             then this setting not required.
           </dd>
         </dl>
+        <p>
+          Note: The <code>managed</code> attribute is only used with PCI devices
+          and is ignored by all the other device types, thus setting
+          <code>managed</code> explicitly with other than PCI device has the same
+          effect as omitting it.
+        </p>
       </dd>
       <dt><code>source</code></dt>
       <dd>The source element describes the device as seen from the host using