nixos/networking-interfaces: clean up networking.sits

This change rework a bit the documentation on networking.sits to explain
what they actually are. In fact, there are three different protocols
being collectively called "SIT", which itself is a nonstandard term.

nixos/modules/tasks/network-interfaces-systemd.nix

@@ -438,7 +438,7 @@ in
                   // (optionalAttrs (sit.ttl != null) {
                     TTL = sit.ttl;
                   })
-                  // (optionalAttrs (sit.encapsulation != null) (
+                  // (optionalAttrs (sit.encapsulation.type != "6in4") (
                     {
                       FooOverUDP = true;
                       Encapsulation = if sit.encapsulation.type == "fou" then "FooOverUDP" else "GenericUDPEncapsulation";