Add description of type definitions

If a type definition has a doc domment, it wasn't included as description
of that type.

Example:

```
/// A forum thread
record thread {
title!: title,
}

/// Thread title
def type title = string constrained {
maxLength: 200,
minLength: 1,
};
```

Would lead to just

```json
"title": {
"type": "string",
"maxLength": 200,
"minLength": 1
}
```

With this commit it contains the `Thread title` as description:

```
"title": {
"type": "string",
"description": "Thread title",
"maxLength": 200,
"minLength": 1
}