Docs suggestion: document the `HashCheckpoint` methods that only work with mainnet

[thunderbiscuit]

I'm working on the update of the Kyoto example in the book of bdk, and after updating the ScanType::Recovery argument on the build_with_wallet method I realized I was using methods that don't work on Signet.

I think the rustdoc could mention this for the HashCheckpoint::taproot_activation and the HashCheckpoint::segwit_activation methods.

[rustaceanrob]

Because taproot and segwit are active since genesis on both signet and testnet4, I opted to not pass a Network to each of these calls, but would passing a Network be preferable in your opinion? Otherwise I'll add a note on the rustdoc