Add comment for the class

https://github.com/linksplatform/Data.Doublets/blob/239271a460e87c6b0e93212d13387b37373acf0e/csharp/Platform.Data.Doublets/Sequences/Frequencies/Counters/TotalMarkedSequenceSymbolFrequencyCounter.cs#L8
The comment should be like this:
```
Counts total amount of sequences in which the symbol (link) is used.
```
If it is required - rename the class to match the logic from comment.