[a11y] Form a11y docs

[dpersing]

WHY are these changes introduced?

Adds accessibility guidance for the form component, to appear in polaris-react docs and in the style guide.

WHAT is this pull request doing?

• Adds accessibility information to the component README.md
• Adds an entry to the documentation section of 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/form (web only)

Screenshots

Web

[chloerice]

Use the submit prop to set the type attribute to submit instead.

Here I'm unclear whether you mean the submit prop on Button or Form 🤔

Suggested change

	By default, any buttons that you add to the form are given a `type` attribute set to `button`. Use the `submit` prop to set the `type` attribute to `submit` instead. The form can have only one submit button, and it must be at the end of the form.
	Forms can have only one submit button and it must be at the end of the form. By default, all buttons contained by the form component are given a `type` attribute set to `button`. Set the `submit` prop on the button component to set its `type` attribute to `submit` instead.

[dpersing]

@chloerice Gotcha! Would this be clearer?

Forms can have only one submit button and it must be at the end of the form. By default, buttons added to the form are given a type attribute set to button to avoid conflicts. To make a button the submit button instead, set the submit prop on the button.