Merge bitcoindevkit/bdk#1188: doc: Improve TxGraph & co docs

Merge bitcoindevkit/bdk#1188: doc: Improve TxGraph & co docs

0adff9c35f109a15740212b4dce74c03950c85ed doc: Improve TxGraph & co docs (Daniela Brozzoni)

Pull request description:

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  notmandatory:
    Re-ACK 0adff9c35f109a15740212b4dce74c03950c85ed

Tree-SHA512: 83adffeddf6c8ddb7b5e57a0fa2e5cffaf75cdeea59be0a6c5e94dcd5a7f98328db8e2a620edc753e60ea3382282908c75d34783a280348cfd105a37982c762b