Merge bitcoindevkit/bdk#753: Improve docs regarding PSBT finalization

Merge bitcoindevkit/bdk#753: Improve docs regarding PSBT finalization

8963e8c9f445f4069262edffd15cbd971c0adc6b Improve docs w.r.t. PSBT finalization (Elias Rohrer)

Pull request description:

  ### Description

  This PR includes just a few tiny changes to the docs trying to make it a bit more clear what PSBT finalization is and what to expect.

  ### Notes to the reviewers

  ### Changelog notice

  ### 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:
  afilini:
    ACK 8963e8c9f445f4069262edffd15cbd971c0adc6b

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