docs: document rng backend path restrictions of older libvirt

Commit 67f2b72 removed the path restrictions for rng backend,
along with the documentation of the restriction.

Restore the documentation for users still using older libvirt.

https://bugzilla.redhat.com/show_bug.cgi?id=1074464

diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index dd9f153336ebebc992c18d200b0c272cbc798380..7a42b8e00563cea5f2ee805644c44c1c10579850 100644 (file)
--- a/docs/formatdomain.html.in
+++ b/docs/formatdomain.html.in
@@ -6360,9 +6360,10 @@ qemu-kvm -net nic,model=? /dev/null
             <p>
               This backend type expects a non-blocking character device as
               input. The file name is specified as contents of the
-              <code>backend</code> element. When no file name is specified
-              the hypervisor default is used. For qemu, the default is
-              /dev/random
+              <code>backend</code> element. <span class='since'>Since 1.3.4</span>
+              any path is accepted. Before that /dev/random and /dev/hwrng were
+              the only accepted paths. When no file name is specified the hypervisor
+              default is used. For qemu, the default is /dev/random
             </p>
           </dd>
           <dt><code>egd</code></dt>