docs: add info about IPv6 and DNS requirements for NTP/autoupdater

docs/user/faq.rst

 Frequently Asked Questions
 ==========================
 
+.. _faq-dns:
+
+DNS does not work on the nodes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Gluon nodes will ignore the DNS server on the WAN port for evenything except
+the mesh VPN, which can lead to confusion.
+
+All normal services on the nodes exclusively use the DNS server on the mesh
+interface. This DNS server must be announced in router advertisements (using
+*radvd* or a similar software) from one or more central servers in meshes based
+on *batman-adv*. If your mesh does not have global IPv6 connectivity, you can setup
+your *radvd* not to announce a default route by setting the *default lifetime* to 0;
+in this case, the *radvd* is only used to announce the DNS server.

docs/user/site.rst

@@ -46,6 +46,9 @@ ntp_server
 
        ntp_servers = {'1.ntp.services.ffeh','2.ntp.services.ffeh'}
 
+    This NTP servers must be reachable via IPv6 from the nodes. If you don't want to set an IPv6 address
+    explicitly, but use a hostname (which is recommended), see also the :ref:`FAQ <faq-dns>`.
+
 opkg : optional
     ``opkg`` package manager configuration.
 
@@ -249,6 +252,9 @@ autoupdater : package
         }
       }
 
+    All configured mirrors must be reachable from the nodes via IPv6. If you don't want to set an IPv6 address
+    explicitly, but use a hostname (which is recommended), see also the :ref:`FAQ <faq-dns>`.
+
 roles : optional
     Optional role definitions. Nodes will announce their role inside the mesh.
     This will allow in the backend to distinguish between normal, backbone and