At a minimum, help text should note this. This is true for things like addwitnessaddress especially.
{"changeAddress", UniValueType(UniValue::VSTR)},
                {"changePosition", UniValueType(UniValue::VNUM)},
                {"includeWatching", UniValueType(UniValue::VBOOL)},
                {"lockUnspents", UniValueType(UniValue::VBOOL)},
                {"reserveChangeKey", UniValueType(UniValue::VBOOL)}, 
                {"feeRate", UniValueType()}, 
                {"subtractFeeFromOutputs", UniValueType(UniValue::VARR)},
                {"replaceable", UniValueType(UniValue::VBOOL)},
                {"conf_target", UniValueType(UniValue::VNUM)},
                {"estimate_mode", UniValueType(UniValue::VSTR)},
            },
            true, true);
        if (options.exists("changeAddress")) {
            CTxDestination dest = DecodeDestination(options["changeAddress"].get_str());
            if (!IsValidDestination(dest)) {
                throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "changeAddress must be a valid bitcoin address");
            }
            coinControl.destChange = dest;
        }
        if (options.exists("changePosition"))
            changePosition = options["changePosition"].get_int();
        if (options.exists("includeWatching"))
            coinControl.fAllowWatchOnly = options["includeWatching"].get_bool();
        if (options.exists("lockUnspents"))
            lockUnspents = options["lockUnspents"].get_bool();
        if (options.exists("feeRate"))
        {
            coinControl.m_feerate = CFeeRate(AmountFromValue(options["feeRate"]));
            coinControl.fOverrideFeeRate = true;
        }