networking/bonds: fix examples

After the change of the bonding options, the examples were not quite correct.
The diff is over-the top because the new `let` needs everything indented.

Also add a small docstring to the `networkd` attr in the networking test.

nixos/tests/networking.nix

@@ -1,4 +1,6 @@
-{ system ? builtins.currentSystem, networkd }:
+{ system ? builtins.currentSystem
+# bool: whether to use networkd in the tests
+, networkd }:
 
 with import ../lib/testing.nix { inherit system; };
 with pkgs.lib;