[a11y] Account connection and setting toggle a11y docs

[dpersing]

WHY are these changes introduced?

Adds accessibility guidance for the account connection and setting toggle components, to appear in polaris-react docs and in the style guide.

WHAT is this pull request doing?

• Adds accessibility documentation for the account connection component (web only)
• Adds accessibility documentation for the setting toggle component (web only)
• Adds an entry to UNRELEASED.md

How to 🎩

1. Check out master from polaris-styleguide to get the changes that support the accessibility section.
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/actions/account-connection (web only)
• https://polaris.myshopify.io/components/actions/setting-toggle (web only)

Screenshots

Account connection

Setting toggle

[sadiesaurus]

One small comment to change future tense to present tense. Looks good!