JSDoc comments in typings to enhance usability of props

- `downshift` version: 5.4.0

**Problem description**:

Currently, all descriptions of props are found through Github. They are nice and all, but it is a hassle to access the repo every time I need to read further up on the props.

**Suggested solution**:

Add all descriptions to `index.d.ts` from the README's in a JSDoc-like format (e.g. typedoc), thus having the descriptions readily available through your IDE/editor of choice

Here is an example of what it may look like in VSCode when a line was added.