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/modules/tasks/network-interfaces.nix

@@ -560,15 +560,20 @@ in
 
     };
 
-    networking.bonds = mkOption {
+    networking.bonds =
+      let
+        driverOptionsExample = {
+          miimon = "100";
+          mode = "active-backup";
+        };
+      in mkOption {
         default = { };
         example = literalExample {
           bond0 = {
             interfaces = [ "eth0" "wlan0" ];
-          miimon = 100;
-          mode = "active-backup";
+            driverOptions = driverOptionsExample;
           };
-        fatpipe.interfaces = [ "enp4s0f0" "enp4s0f1" "enp5s0f0" "enp5s0f1" ];
+          anotherBond.interfaces = [ "enp4s0f0" "enp4s0f1" "enp5s0f0" "enp5s0f1" ];
         };
         description = ''
           This option allows you to define bond devices that aggregate multiple,
@@ -590,11 +595,7 @@ in
             driverOptions = mkOption {
               type = types.attrsOf types.str;
               default = {};
-            example = literalExample {
-              interfaces = [ "eth0" "wlan0" ];
-              miimon = 100;
-              mode = "active-backup";
-            };
+              example = literalExample driverOptionsExample;
               description = ''
                 Options for the bonding driver.
                 Documentation can be found in

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;