At a minimum, help text should note this. This is true for things like addwitnessaddress especially.
int pos = subtractFeeFromOutputs[idx].get_int();
        if (setSubtractFeeFromOutputs.count(pos))
            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, duplicated position: %d", pos));
        if (pos < 0)
            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, negative position: %d", pos));
        if (pos >= int(tx.vout.size()))
            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, position too large: %d", pos));
        setSubtractFeeFromOutputs.insert(pos);
    }
    CAmount nFeeOut;
    std::string strFailReason;
    if (!pwallet->FundTransaction(tx, nFeeOut, changePosition, strFailReason, lockUnspents, setSubtractFeeFromOutputs, coinControl)) {
        throw JSONRPCError(RPC_WALLET_ERROR, strFailReason);
    }
    UniValue result(UniValue::VOBJ);
    result.push_back(Pair("hex", EncodeHexTx(tx)));
    result.push_back(Pair("changepos", changePosition));
    result.push_back(Pair("fee", ValueFromAmount(nFeeOut)));
    return result;
}
UniValue bumpfee(const JSONRPCRequest& request)
{
    CWallet * const pwallet = GetWalletForJSONRPCRequest(request);
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp))
        return NullUniValue;
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 2) {
        throw std::runtime_error(
            "bumpfee \"txid\" ( options ) \n"
            "\nBumps the fee of an opt-in-RBF transaction T, replacing it with a new transaction B.\n"
            "An opt-in RBF transaction with the given txid must be in the wallet.\n"