Documentation errors/improvements
Hey Mathieu and team,

Took the new API for a test drive today. Major improvement from the old way of doing things! The documentation needs some tweaks here and there to match, though. Some of the issues I noticed were addressed in other Issues, so I'll just point out the things I didn't see mentioned elsewhere:

- `Layer.parent` is mutable, and moves layers between different groups/parents. I had to guess to find this one.
- `Text.fragments` contains a `range` property that tells what range of text within the text layer the fragment encompasses.
- `Rectangle` is documented as being defined on `Sketch.Document` when it is in fact on `Sketch` (which makes more sense to me). This one puzzled me a bit till I did some testing.

That's it for now. Once again, great work - can't wait to see what else is in store.