Merge bitcoindevkit/bdk#1158: doc(bdk): Clarify the absolute_fee docs

Merge bitcoindevkit/bdk#1158: doc(bdk): Clarify the absolute_fee docs

0ecc0280c032301dc638e75e54a8691d66d50274 doc(bdk): Clarify the absolute_fee, fee_rate docs (Daniela Brozzoni)

Pull request description:

  Fixes #1066

  ### 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:
    ACK 0ecc0280c032301dc638e75e54a8691d66d50274

Tree-SHA512: 152e48b86c21d4fad711abf76dd8fdc0e8955030331005c1ba6ff0c866c52870161f91ec740838f8238c5ad1c620e06212099308a55d130699cb18e4666e3f3f