docs: use IPv6 addresses in range reserved for documentation

Someone in an IRC channel or an email pointed out a few days ago that
the examples of IPv6 addresses in the libvirt documentation were not
in the officially reserved "documentation" range. This addresses their
concern.

docs/formatnetwork.html.in

@@ -350,7 +350,7 @@
             <host mac="00:16:3e:3e:a9:1a" name="bar.example.com" ip="192.168.122.11" />
           </dhcp>
         </ip>
-        <ip family="ipv6" address="2001:8794:ca2:2::1" prefix="64" />
+        <ip family="ipv6" address="2001:db8:ca2:2::1" prefix="64" />
       &lt;/network&gt;</pre>
 
     <dl>
@@ -490,7 +490,7 @@
             &lt;range start="192.168.122.2" end="192.168.122.254" /&gt;
           &lt;/dhcp&gt;
         &lt;/ip&gt;
-        &lt;ip family="ipv6" address="2001:8794:ca2:2::1" prefix="64" /&gt;
+        &lt;ip family="ipv6" address="2001:db8:ca2:2::1" prefix="64" /&gt;
       &lt;/network&gt;</pre>
 
     <h3><a name="examplesRoute">Routed network config</a></h3>
@@ -514,7 +514,7 @@
             &lt;range start="192.168.122.2" end="192.168.122.254" /&gt;
           &lt;/dhcp&gt;
         &lt;/ip&gt;
-        &lt;ip family="ipv6" address="2001:8794:ca2:2::1" prefix="64" /&gt;
+        &lt;ip family="ipv6" address="2001:db8:ca2:2::1" prefix="64" /&gt;
       &lt;/network&gt;</pre>
 
     <h3><a name="examplesPrivate">Isolated network config</a></h3>
@@ -536,7 +536,7 @@
             &lt;range start="192.168.152.2" end="192.168.152.254" /&gt;
           &lt;/dhcp&gt;
         &lt;/ip&gt;
-        &lt;ip family="ipv6" address="2001:8794:ca2:3::1" prefix="64" /&gt;
+        &lt;ip family="ipv6" address="2001:db8:ca2:3::1" prefix="64" /&gt;
       &lt;/network&gt;</pre>
 
     <h3><a name="examplesBridge">Using an existing host bridge</a></h3>