docs: Drop mention that WWN for disks must be unique

For multipath disks it might be useful to have the same WWN for multiple
disks. It's the users choice to do so. Since we dropped the check that
disallows using duplicate WWNs drop the docs as well.

https://bugzilla.redhat.com/show_bug.cgi?id=1464975

docs/formatdomain.html.in

@@ -3184,8 +3184,7 @@
       <dt><code>wwn</code></dt>
       <dd>If present, this element specifies the WWN (World Wide Name)
         of a virtual hard disk or CD-ROM drive. It must be composed
-        of 16 hexadecimal digits and must be unique (at least among
-        disks of a single domain)
+        of 16 hexadecimal digits.
         <span class='since'>Since 0.10.1</span>
       </dd>
       <dt><code>vendor</code></dt>