message: rpc: document optional fields for getchaintxstats result
diff: @@ -1539,9 +1539,9 @@ static RPCHelpMan getchaintxstats()
  {RPCResult::Type::STR_HEX, "window_final_block_hash", "The hash of the final block in the window"},
  {RPCResult::Type::NUM, "window_final_block_height", "The height of the final block in the window."},
  {RPCResult::Type::NUM, "window_block_count", "Size of the window in number of blocks"},
  - {RPCResult::Type::NUM, "window_tx_count", "The number of transactions in the window. Only returned if \"window_block_count\" is > 0"},
  - {RPCResult::Type::NUM, "window_interval", "The elapsed time in the window in seconds. Only returned if \"window_block_count\" is > 0"},
  - {RPCResult::Type::NUM, "txrate", "The average rate of transactions per second in the window. Only returned if \"window_interval\" is > 0"},
  + {RPCResult::Type::NUM, "window_tx_count", /* optional */ true, "The number of transactions in the window. Only returned if \"window_block_count\" is > 0"},
  + {RPCResult::Type::NUM, "window_interval", /* optional */ true, "The elapsed time in the window in seconds. Only returned if \"window_block_count\" is > 0"},
  + {RPCResult::Type::NUM, "txrate", /* optional */ true, "The average rate of transactions per second in the window. Only returned if \"window_interval\" is > 0"},
  }},
  RPCExamples{
  HelpExampleCli("getchaintxstats", "")
  