This function is strange. Looks like the implementation is of low quality.

```
bool isDeterministic() const override { return false; }
bool isDeterministicInScopeOfQuery() const override { return false; }
```
Why it is not deterministic? Missing a comment.

```
String convertToJson(const PODArray<String> & variant_names, const Variants & variants)
{
    FormatSettings settings;

    WriteBufferFromOwnString buf;

    writeCString("{\"data\":[", buf);
    for (size_t i = 0; i < variants.size(); ++i)
    {
        writeCString("{\"variant_name\":", buf);
        writeJSONString(variant_names[i], buf, settings);
        writeCString(",\"x\":", buf);
        writeText(variants[i].x, buf);
        writeCString(",\"y\":", buf);
        writeText(variants[i].y, buf);
        writeCString(",\"beats_control\":", buf);
        writeText(variants[i].beats_control, buf);
        writeCString(",\"to_be_best\":", buf);
        writeText(variants[i].best, buf);
        writeCString("}", buf);
        if (i != variant_names.size() -1)
            writeCString(",", buf);
    }
    writeCString("]}", buf);

    return buf.str();
}
```
It returns unstructured data? :scream: 

As first step we can remove documentation for this function.
CC @nikitamikhaylov 
registerFunctionsMap(factory);
#if !defined(ARCADIA_BUILD)
    registerFunctionsBitmap(factory);
#endif
    registerFunctionsBinaryRepr(factory);
    registerFunctionsCoding(factory);
    registerFunctionsCodingUUID(factory);
    registerFunctionChar(factory);
    registerFunctionsComparison(factory);
    registerFunctionsConditional(factory);
    registerFunctionsConversion(factory);
    registerFunctionCastOrDefault(factory);
    registerFunctionsDateTime(factory);
    registerFunctionsEmbeddedDictionaries(factory);
    registerFunctionsExternalDictionaries(factory);
    registerFunctionsExternalModels(factory);
    registerFunctionsFormatting(factory);
    registerFunctionsHashing(factory);
    registerFunctionsHigherOrder(factory);
    registerFunctionsLogical(factory);
    registerFunctionsMiscellaneous(factory);
    registerFunctionsRandom(factory);
    registerFunctionsReinterpret(factory);
    registerFunctionsRound(factory);
    registerFunctionsString(factory);
    registerFunctionsStringArray(factory);
    registerFunctionsStringSearch(factory);
    registerFunctionsStringRegexp(factory);
    registerFunctionsStringSimilarity(factory);
    registerFunctionNgrams(factory);