Add comment for listToSequenceLinkConverter argument

https://github.com/linksplatform/Data.Doublets/blob/239271a460e87c6b0e93212d13387b37373acf0e/csharp/Platform.Data.Doublets/Unicode/StringToUnicodeSequenceConverter.cs#L18

In the comment add the list of possible values (inherited from LinksListToSequenceConverterBase):
* OptimalVariantConverter
* BalancedVariantConverter.cs
* CompressingConverter.cs