message: rpc: Document getwalletinfo's unlocked_until field as optional
diff: @@ -2334,7 +2334,7 @@ static UniValue getwalletinfo(const JSONRPCRequest& request)
  {RPCResult::Type::NUM_TIME, "keypoololdest", "the " + UNIX_EPOCH_TIME + " of the oldest pre-generated key in the key pool. Legacy wallets only."},
  {RPCResult::Type::NUM, "keypoolsize", "how many new keys are pre-generated (only counts external keys)"},
  {RPCResult::Type::NUM, "keypoolsize_hd_internal", "how many new keys are pre-generated for internal use (used for change outputs, only appears if the wallet is using this feature, otherwise external keys are used)"},
  - {RPCResult::Type::NUM_TIME, "unlocked_until", "the " + UNIX_EPOCH_TIME + " until which the wallet is unlocked for transfers, or 0 if the wallet is locked"},
  + {RPCResult::Type::NUM_TIME, "unlocked_until", /* optional */ true, "the " + UNIX_EPOCH_TIME + " until which the wallet is unlocked for transfers, or 0 if the wallet is locked (only present for passphrase-encrypted wallets)"},
  {RPCResult::Type::STR_AMOUNT, "paytxfee", "the transaction fee configuration, set in " + CURRENCY_UNIT + "/kB"},
  {RPCResult::Type::STR_HEX, "hdseedid", /* optional */ true, "the Hash160 of the HD seed (only present when HD is enabled)"},
  {RPCResult::Type::BOOL, "private_keys_enabled", "false if privatekeys are disabled for this wallet (enforced watch-only wallet)"},
  