Merge bitcoindevkit/bdk-cli#364: docs: list supported network values in CLI help

Merge bitcoindevkit/bdk-cli#364: docs: list supported network values in CLI help

405a0e3ce15ea437af01e509cc98d9576b22e81f docs: list supported network values in CLI help (Martino Salvetti)

Pull request description:

  ### Description

  List the accepted --network values in CLI help.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk-cli/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  vadim-anfv:
    tACK 405a0e3ce15ea437af01e509cc98d9576b22e81f; successfully ran `just pre-push`; thanks!
  tvpeter:
    ACK 405a0e3ce15ea437af01e509cc98d9576b22e81f

Tree-SHA512: e34dc314aa78a64b6c185eacbf089a1e21707483b583c5a2fd8a2ca59b54f4e1b4ec3d2650f4215923811516e566c7a818ae04f03cc23c7f473a51f9fd801eec