Language service: meaning of encoding in classifiers

The following code : https://github.com/Microsoft/TypeScript/blob/12c6a7400dffc10c41f9245da64fbd5beeceb3dc/src/services/services.ts#L7843-L7847

``` ts
if (length > 0) {
result.spans.push(start);
result.spans.push(length);
result.spans.push(classification);
}
```

I also found the comment `// Encoded as triples of [start, length, ClassificationType].` in the source. It was completely unobvious till I reviewed the code and can break on API consumers if new things are needed. Would be better if it was moved into `{start,length,classification}` datastructure, but I guess they did it for performance reasons :rose:

Start by reading the linked range in src/services/services.ts and the nearby comment describing triples of [start, length, ClassificationType]. Determine whether this repository should explain the classifier encoding or whether the concern belongs upstream; done requires an agreed scope and a clear documentation change.

Written by the indexing model from the issue text.