Merge bitcoin#29302: wallet: clarify replaced_by_txid and replaces_txid in help output

ff54314 wallet: clarify replaced_by_txid and replaces_txid in help output (marco)

Pull request description:

  Resolves issue bitcoin#27781

ACKs for top commit:
  achow101:
    ACK ff54314
  ryanofsky:
    Code review ACK ff54314. Seems like a helpful clarification

Tree-SHA512: b13a0e24505dfaee083467ac6f357b96460b5d1841dc29c4df4a503c290d379cef3d50fcc76f33bbc95741f484dd9d2461b0c2e8bdebf57a8a72edfbeece2a79

Author: achow101

src/wallet/rpc/transactions.cpp

@@ -415,8 +415,8 @@ static std::vector<RPCResult> TransactionDescriptionString()
            {
                {RPCResult::Type::STR_HEX, "txid", "The transaction id."},
            }},
-           {RPCResult::Type::STR_HEX, "replaced_by_txid", /*optional=*/true, "The txid if this tx was replaced."},
-           {RPCResult::Type::STR_HEX, "replaces_txid", /*optional=*/true, "The txid if the tx replaces one."},
+           {RPCResult::Type::STR_HEX, "replaced_by_txid", /*optional=*/true, "Only if 'category' is 'send'. The txid if this tx was replaced."},
+           {RPCResult::Type::STR_HEX, "replaces_txid", /*optional=*/true, "Only if 'category' is 'send'. The txid if this tx replaces another."},
            {RPCResult::Type::STR, "to", /*optional=*/true, "If a comment to is associated with the transaction."},
            {RPCResult::Type::NUM_TIME, "time", "The transaction time expressed in " + UNIX_EPOCH_TIME + "."},
            {RPCResult::Type::NUM_TIME, "timereceived", "The time received expressed in " + UNIX_EPOCH_TIME + "."},