Merge bitcoin/bitcoin#18448: rpc: fix/add missing RPCExamples for "Util" RPCs

ea98d9c2eff86e6537f35ac4381ac169daacde36 rpc: fix/add missing RPCExamples for "Util" RPCs (Sebastian Falbesoner)

Pull request description:

  Similar to https://github.com/bitcoin/bitcoin/pull/18398, this PR gives the RPCExamples in the RPC category "Util" (that currently contains `createmultisig`, `deriveaddresses`, `estimatesmartfee`, `getdescriptorinfo`, `signmessagewithprivkey`, `validateaddress`, `verifymessage`) some love by fixing one broken and adding three missing examples:
  - fixed `HelpExampleRpc` for `createmultisig` (disturbing escape characters and quotation marks)
  - added missing `HelpExampleRpc` for
      -  `deriveaddresses` (also put descriptor in a new string constant)
      - `estimatesmartfee`
      - `getdescriptorinfo` (also put descriptor in a new string constant)

  Output for `createmultisig` example on the master branch:
  ```
  $ curl --user __cookie__ --data-binary '{"jsonrpc": "1.0", "id": "curltest", "method": "createmultisig", "params": [2, "[\"03789ed0bb717d88f7d321a368d905e7430207ebbd82bd342cf11ae157a7ace5fd\",\"03dbc6764b8884a92e871274b87583e6d5c2a58819473e17e107ef3f6aa5a61626\"]"]}' -H 'content-type: text/plain;' http://127.0.0.1:8332/
  Enter host password for user '__cookie__':
  {"result":null,"error":{"code":-1,"message":"JSON value is not an array as expected"},"id":"curltest"}
  ```

  Output for `createmultisig` example on the PR branch:
  ```
  $ curl --user __cookie__ --data-binary '{"jsonrpc": "1.0", "id": "curltest", "method": "createmultisig", "params": [2, ["03789ed0bb717d88f7d321a368d905e7430207ebbd82bd342cf11ae157a7ace5fd","03dbc6764b8884a92e871274b87583e6d5c2a58819473e17e107ef3f6aa5a61626"]]}' -H 'content-type: text/plain;' http://127.0.0.1:8332/
  Enter host password for user '__cookie__':
  {"result":{"address":"3QsFXpFJf2ZY6GLWVoNFFd2xSDwdS713qX","redeemScript":"522103789ed0bb717d88f7d321a368d905e7430207ebbd82bd342cf11ae157a7ace5fd2103dbc6764b8884a92e871274b87583e6d5c2a58819473e17e107ef3f6aa5a6162652ae","descriptor":"sh(multi(2,03789ed0bb717d88f7d321a368d905e7430207ebbd82bd342cf11ae157a7ace5fd,03dbc6764b8884a92e871274b87583e6d5c2a58819473e17e107ef3f6aa5a61626))#4djp057k"},"error":null,"id":"curltest"}
  ```

ACKs for top commit:
  jonatack:
    ACK ea98d9c2eff86e6537f35ac4381ac169daacde36 looked at the code, rebased to master, ran the helps, did not try running the added json-rpc examples

Tree-SHA512: d6ecb6da66f19517065453357d210102e2cc9f1f8037aeb6a9177ff036d0c21773dddf5e0acdbc71edbbde3026e4d1e7ce7c0935cd3e023c60f34e1b173b3299

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