rpc: document RPC_TRANSACTION_ALREADY_IN_CHAIN exception

It is not documented that if you attempt to send a transaction
which already exists in a block, an RPC_TRANSACTION_ALREADY_IN_CHAIN
exception will be raised. This should be documented so that developers
are aware that this exception is raised.

Author: jarolrod

src/rpc/rawtransaction.cpp

@@ -816,10 +816,11 @@ static RPCHelpMan sendrawtransaction()
 {
     return 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 sendrawtransaction\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()),