docs: mesh-vpn: recommend better default mtu

docs/site-example/site.conf

@@ -100,7 +100,7 @@
 
   mesh_vpn = {
     -- enabled = true,
-    mtu = 1280,
+    mtu = 1312,
 
     fastd = {
       -- Refer to http://fastd.readthedocs.org/en/latest/ to better understand

docs/user/site.rst

@@ -223,7 +223,7 @@ mesh_vpn
 
       mesh_vpn = {
         -- enabled = true,
-        mtu = 1280,
+        mtu = 1312,
 
         fastd = {
           methods = {'salsa2012+umac'},