Improve /domainsnapshot/disks/disk@snapshot doc

The previous description was a bit confusing.

docs/formatsnapshot.html.in

@@ -146,8 +146,9 @@
             the <a href="formatdomain.html#elementsDisks">disk
             devices</a> specified for the domain at the time of the
             snapshot.  The attribute <code>snapshot</code> is
-            optional, and has the same values of the disk device
-            element for a domain
+            optional, and the possible values are the same as the
+            <code>snapshot<code> attribute for
+             <a href="formatdomain.html#elementsDisks">disk devices</a>
             (<code>no</code>, <code>internal</code>,
             or <code>external</code>).  Some hypervisors like ESX
             require that if specified, the snapshot mode must not