Missing custom arrows from API documentation

I'm mostly developing using [the API docs](https://react-slick.neostack.com/docs/api) as the main source to understand this lib.

One of the things that I was worried about is that I couldn't customize arrow components. There were only mentions about `dotsClass`, none about previous and next buttons. So I chose to not show arrows altogether.

Fortunately, I discovered that it is possible when [browsing the demo section](https://react-slick.neostack.com/docs/example/custom-arrows). I thought it was only misdocumented at first, but then I [implemented at the playground](https://codesandbox.io/s/91xl6rjvzo) and it worked :smile:

I think it would be a lot easier for newcomers to understand the lib if all the props were properly documented. I searched for some related issues and it seems I'm not the only one confused about this.
https://github.com/akiran/react-slick/issues/111
https://github.com/akiran/react-slick/issues/1374

This documentation improvement would resolve issues before they appear, and make it easier for this lib to be used.