Merge bitcoindevkit/bdk#1806: fix(docs): `merge_chains` outdated documentation

5647241f45b29fe5f7ad5aa5a94c0c4f72f9adea fix(docs): `merge_chains` documentation (Leonardo Lima)

Pull request description:

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  ### Description

  It's a minor documentation fix, as reported during the audit and referenced in bitcoindevkit/bdk_wallet#59.

  <!-- Describe the purpose of this PR, what's being adding and/or fixed -->

  ### Notes to the reviewers

  I'm unsure if anything else / any other explanation should be included in the documentation. Let me know if you think more context should be added to it.

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  of the PR were done in a specific way -->

  ### Changelog notice

  - Update `bdk_chain::local_chain::merge_chains` documentation.

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  <!-- See https://keepachangelog.com/en/1.0.0/ for examples -->

  ### 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:
  evanlinjin:
    self-ACK 5647241f45b29fe5f7ad5aa5a94c0c4f72f9adea

Tree-SHA512: a70a67eab9c8fa0c60a674efed82ddeb9bc9b69763fefaa38a9058d7a1e67c06045c3e5a31aaf0d48c0a599d03bac0a3a86530cc250acefea9497572c01b8b0e