docs: domain: /dev/urandom isn't a valid rng patch

Only /dev/random and /dev/hwrng are accepted, list them explicitly.

docs/formatdomain.html.in

@@ -4666,7 +4666,7 @@ qemu-kvm -net nic,model=? /dev/null
       <dd>
         <p>
           This backend type expects a non-blocking character device as input.
-          Examples of such devices are /dev/random and /dev/urandom. The file
+          The only accepted paths are /dev/random and /dev/hwrng. The file
           name is specified as contents of the <code>backend</code> element.
           When no file name is specified the hypervisor default is used.
         </p>