Merge bitcoindevkit/bdk#969: Update documentation for fee_rate and fee_absolute metho...

e6519e3a52a1ab8023b5fdf423d8f748a3b0260d Enhance the documentation for the fee_rate and fee_absolute methods. (Jon Marrs)

Pull request description:

  ### Description

  This pr helps solve this issue: https://github.com/bitcoindevkit/bdk/issues/856

  I added documentation to the fee_rate() method to describe the units as either satoshis/vbyte (sats/vbyte) or satoshis/kwu (sats/kwu), depending on the FeeRate type.

  I also added documentation to the fee_absolute() method to clarify that the fee is determined by whichever method (fee_rate or fee_absolute) was called last, as the FeePolicy is an enum, and FeeRate/FeeAmount are mutually exclusive.

  ### Notes to the reviewers

  I thought it would be helpful to provide documentation to alleviate confusion over the fee_rate method and the fee_absolute method.

  ### 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)

  #### Bugfixes:

  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  danielabrozzoni:
    ACK e6519e3a52a1ab8023b5fdf423d8f748a3b0260d

Tree-SHA512: 59f62d1d1f8355e6353c6a2550e464732975c86c767648adc9143f2b3a9b894a90536a30a33e9de7efbe53f16392ec2e19008d884fb65ef037edae64a3cb6970