Merge bitcoindevkit/bdk#957: Documentation regarding absolute_fee and fee_rate updated

Merge bitcoindevkit/bdk#957: Documentation regarding absolute_fee and fee_rate updated

10b4b6c665d58321436413d1d9c0486e7d74c5ec Documentations regarding absolute_fee and fee_rate updated (TATHAGATA ROY)

Pull request description:

  ### Description
  This pr solves this issue #856
  Updated the documentation for absolute fee and fee rate in BDK docs

  #### 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 10b4b6c665d58321436413d1d9c0486e7d74c5ec

Tree-SHA512: 5b5f360b742313443368dc0b75275c38888cb358545661f7d9c78f3290789c50ffa9830e18473055d8e3da544c7d9527b023a1acdb28eaf862b24c892b23e9c4