nixos/manual: add hint to interactive testing about Internet access

verified with following minimal test, embedded in a flake for ease of
execution:
```nix
{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/master";
  outputs = { nixpkgs, ... }:
    let
      pkgs = import nixpkgs { system = "x86_64-linux"; };
    in
    {
      nixosTests.x86_64-linux.internet = pkgs.nixosTest {
        name = "internet";
        nodes.machine = { };
        # only succeeds in interactive mode
        testScript = ''
          machine.wait_for_unit("default.target")
          print(machine.succeed("curl -4 https://nixos.org"))
        '';
      };
    };
}
```

Launch this test interactively as described in the manual & run
`test_script()` to verify that the VM has Internet access.

nixos/doc/manual/development/running-nixos-tests-interactively.section.md

@@ -10,6 +10,12 @@ $ ./result/bin/nixos-test-driver
 >>>
 ```
 
+::: {.note}
+By executing the test driver in this way,
+the VMs executed may gain network & Internet access via their backdoor control interface,
+typically recognized as `eth0`.
+:::
+
 You can then take any Python statement, e.g.
 
 ```py