Merge bitcoin#20941: rpc: document RPC_TRANSACTION_ALREADY_IN_CHAIN exception

74d23bf rpc: document RPC_TRANSACTION_ALREADY_IN_CHAIN exception (Jarol Rodriguez)

Pull request description:

  It is not documented in the `RPCHelpMan` of `sendrawtransaction` that if you attempt to send a transaction which already exists in a block, an `RPC_TRANSACTION_ALREADY_IN_CHAIN` exception will be raised. It is best to make developers aware of this so that it can be properly caught and avoid any headaches.

  Closes dashpay#5638

ACKs for top commit:
  jonatack:
    ACK 74d23bf

Tree-SHA512: d1d5fc242574377c8a76b4ef7b12239996424d8bee186533b5a8fe337bbeb3186e51dbdd28c5eafb982601e44e17b68a7f52db5dd7bc647429f6f95e2de289f6

Author: MarcoFalke

src/rpc/rawtransaction.cpp

@@ -991,10 +991,11 @@ static UniValue signrawtransactionwithkey(const JSONRPCRequest& request)
 UniValue sendrawtransaction(const JSONRPCRequest& request)
 {
     RPCHelpMan{"sendrawtransaction",                "\nSubmit a raw transaction (serialized, hex-encoded) to local node and network.\n"
-                "\nNote that the transaction will be sent unconditionally to all peers, so using this\n"
+                "\nThe transaction will be sent unconditionally to all peers, so using this\n"
                 "for manual rebroadcast may degrade privacy by leaking the transaction's origin, as\n"
                 "nodes will normally not rebroadcast non-wallet transactions already in their mempool.\n"
-                "\nAlso see createrawtransaction and signrawtransactionwithkey calls.\n",
+                "\nA specific exception, RPC_TRANSACTION_ALREADY_IN_CHAIN, may throw if the transaction cannot be added to the mempool.\n"
+                "\nRelated RPCs: createrawtransaction, signrawtransactionwithkey\n",
                 {
                     {"hexstring", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The hex string of the raw transaction"},
                     {"maxfeerate", RPCArg::Type::AMOUNT, /* default */ FormatMoney(DEFAULT_MAX_RAW_TX_FEE_RATE.GetFeePerK()),