b3972bc 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 b3972bc
  kallewoof:
    ACK b3972bc
  ajtowns:
    ACK b3972bc - skimmed code only, looks fine to me

Tree-SHA512: 66c59cdc3c19e8f8a02d3f3f992ff1db06769df63244d4af62629e18aaf4a12b3b7e75e4a0b9f616033cdc4415da046053ba36fede8be145b2dc695b2aa69a02

…for other chains

6fccad7 signet: do not log signet startup messages for other chains (Jon Atack)

Pull request description:

  The following signet startup messages are printed to the debug log immediately on node startup for all chains. This behavior occurs on master as a side effect after the merge of bitcoin#20014. This PR removes the first message and moves the signet derived magic logging to `init.cpp`.
  ```
  $ ./src/bitcoind
  2020-09-30T14:25:15Z Using default signet network
  2020-09-30T14:25:15Z Signet derived magic (message start): 0a03cf40
  2020-09-30T14:25:15Z Bitcoin Core version v0.20.99.0 ...
  ```

ACKs for top commit:
  MarcoFalke:
    ACK 6fccad7
  kallewoof:
    utACK 6fccad7
  hebasto:
    ACK 6fccad7

Tree-SHA512: 33821dce89b24caf7b7c1ecb41e572ecfb26e6958a1316d359ff240e6ef97c4a1f2cf1b4b974596b252815f9df23960ce385c132ebdbc855bbe6123c3b0b003a