Update configuration.nix

[Tracachang]

To clarify the configuration, this update explicitly presents the option to choose Bitcoind Knots.

[jonasnick]

Thanks @Tracachang. I do not think setting bitcoind-knots should be part of an example config because it's an expert option (with subtle trade-offs) and we do not test it.

[Tracachang]

Thanks @jonasnick . Totally understand the hesitation around implying official support or encouraging use of untested options. Still, I think including the bitcoind-knots line (commented out with a clear warning) would be a good way to make the option visible without suggesting it’s maintained. For example:

# Expert option — use Bitcoind Knots instead of Bitcoin Core (untested by nix-bitcoin maintainers):
# services.bitcoind.package = config.nix-bitcoin.pkgs.bitcoind-knots;