[Slider] Add onChange callback signatures to docs

[miclaus]

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

Little commit – something I would have needed.

[mbrookes]

Thanks for adding this. 👍 Would you mind adding the other calbacks in Slider so we can close that one out altogether? 🎉

[miclaus]

I added onDragStart and onDragStop callback events docs.

The other on-* events we're marked with /** @ignore */