YQL-20112: Support experimental documentation

LSP supports the following propoerty of the
completion item, that supports Markdown format:

```
/**
 * A human-readable string that represents a doc-comment.
 */
documentation?: string | MarkupContent;
```

I added docs only for a few items just to check a
look in the UI. Also some docs are shortened.

Documentation is opt-in decorator for a
NameService. Because it is not needed in the
YDB CLI, for example.
commit_hash:b400ed1224be2906b7fb1da29e9c97aa7578d710

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