Merge bitcoin#21311: rpc: document optional fields for getchaintxstats result

73e1f7d rpc: document optional fields for getchaintxstats result (Sebastian Falbesoner)

Pull request description:

  This mini-PR updates the result help of the `getchaintxstats` RPC by showing the following fields as "optional":
  - window_tx_count
  - window_interval
  - txrate

  Help output diff between master and PR branch:
  ```diff
  16,18c16,18
  <   "window_tx_count" : n,                (numeric) The number of transactions in the window. Only returned if "window_block_count" is > 0
  <   "window_interval" : n,                (numeric) The elapsed time in the window in seconds. Only returned if "window_block_count" is > 0
  <   "txrate" : n                          (numeric) The average rate of transactions per second in the window. Only returned if "window_interval" is > 0
  ---
  >   "window_tx_count" : n,                (numeric, optional) The number of transactions in the window. Only returned if "window_block_count" is > 0
  >   "window_interval" : n,                (numeric, optional) The elapsed time in the window in seconds. Only returned if "window_block_count" is > 0
  >   "txrate" : n                          (numeric, optional) The average rate of transactions per second in the window. Only returned if "window_interval" is > 0
  ```

ACKs for top commit:
  0xB10C:
    ACK 73e1f7d

Tree-SHA512: 63c8db3e47a3c2d5564d53c564484b95b656e1e5deca1e9841bc90d122d3c81f02fd2b59313fd913ce81b16f7cc2969fe1dd9d6c3e23628b8ac057ea08f55daa

Author: fanquake

src/rpc/blockchain.cpp

@@ -1669,9 +1669,9 @@ 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", "")