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.