Merge #18467: rpc: Improve documentation and return value of settxfee

38677274f931088218eeb1f258077d3387f39c89 rpc: settxfee respects -maxtxfee wallet setting (Fabian Jahr)
bda84a08a0ac92dff6cadc99cf9bb8c3fadd7e13 rpc: Add documentation for deactivating settxfee (Fabian Jahr)

Pull request description:

  ~~Closes 18315~~

  `settxfee` can be deactivated by passing 0 as the fee. That does not seem to be documented for the user so this PR adds it in the description. The return value of a simple boolean seems also too simplified given the multiple dimensions that this deactivation feature enables. I.e. it does not seem intuitive if the returned boolean shows that the call succeeded or if means that `settxfee` is active. My suggested solution is to change the return value to a JSON object that included the "active" state and the currently set fee rate.

  Examples:
  ```
  $ src/bitcoin-cli settxfee 0.0000000
  {
    "active": false,
    "fee_rate": "0.00000000 BTC/kB"
  }
  $ src/bitcoin-cli settxfee 0.0001
  {
    "active": true,
    "fee_rate": "0.00010000 BTC/kB"
  }
  ```

ACKs for top commit:
  MarcoFalke:
    ACK 38677274f931088218eeb1f258077d3387f39c89, seems useful to error out early instead of later #16257 🕍
  jonatack:
    ACK 38677274f931088218eeb
  meshcollider:
    LGTM, utACK 38677274f931088218eeb1f258077d3387f39c89

Tree-SHA512: 642813b5cf6612abb4b6cb63728081a6bd1659d809e0149c8f56060b6da7253fee989b3b202854f3051df3773c966799af30b612648c466b099f00590f356548

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