Merge #20014: doc: Mention signet in -help output

b3972bca9f74b3e75918676ef029f1964b0622a1 doc: Mention signet in -help output (Hennadii Stepanov)

Pull request description:

  ```
  $ src/bitcoind -help | grep -A 4 -e '-chain=' | head -8
    -chain=<chain>
         Use the chain <chain> (default: main). Allowed values: main, test,
         signet, regtest

    -signet
         Use the signet chain. Equivalent to -chain=signet. Note that the network
         is defined by the -signetchallenge parameter

  ```

  ```
  $ src/bitcoind -help | grep -A 3 -e '-port='
    -port=<port>
         Listen for connections on <port> (default: 8333, testnet: 18333 signet:
         38333, regtest: 18444)

  ```

  ```
  $ src/bitcoind -help | grep -A 3 -e '-rpcport='
    -rpcport=<port>
         Listen for JSON-RPC connections on <port> (default: 8332, testnet:
         18332, signet: 38332, regtest: 18443)

  ```

ACKs for top commit:
  jonatack:
    ACK b3972bca9f74b3e75918676ef029f1964b0622a1
  kallewoof:
    ACK b3972bca9f74b3e75918676ef029f1964b0622a1
  ajtowns:
    ACK b3972bca9f74b3e75918676ef029f1964b0622a1 - skimmed code only, looks fine to me

Tree-SHA512: 66c59cdc3c19e8f8a02d3f3f992ff1db06769df63244d4af62629e18aaf4a12b3b7e75e4a0b9f616033cdc4415da046053ba36fede8be145b2dc695b2aa69a02

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