[a11y] Docs for checkbox, radio button, and choice list

[dpersing]

WHY are these changes introduced?

Adds accessibility guidance for the checkbox, radio button, and choice list components, to appear in polaris-react docs and in the style guide. The choice list documentation points readers to the other components.

WHAT is this pull request doing?

• Adds accessibility documentation for the checkbox README.md
• Adds accessibility documentation for the radio button README.md
• Adds accessibility documentation for the choice list README.md
• Adds an entry to UNRELEASED.md

How to 🎩

1. Check out master from polaris-styleguide
2. In another Terminal tab or window, check out this branch from polaris-react and run the instructions for testing in a consuming project.
3. In the polaris-styleguide tab, run dev up && dev start.
4. View changes after examples and props:

• https://polaris.myshopify.io/components/forms/choice-list (web, iOS, and Android)
• https://polaris.myshopify.io/components/forms/checkbox (web, iOS, and Android)
• https://polaris.myshopify.io/components/forms/radio-button (web, iOS, and Android)

Screenshots

Choice list (web)

Checkbox (web)

Radio button (web)

[kaelig]

Great content, I like how it puts the developer in the merchant's shoes. Just a couple of comments for me!

[dpersing]

Thanks for the keen eyeballs, @kaelig! 😁Tweaks made. 🎉

[sadiesaurus]

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