[a11y] Account connection and setting toggle a11y docs

UNRELEASED.md

@@ -16,6 +16,7 @@ Use [the changelog guidelines](https://git.io/polaris-changelog-guidelines) to f
 
 ### Documentation
 
+- Added accessibility documentation for the account connection and setting toggle components ([#970](https://github.com/Shopify/polaris-react/pull/970))
 - Added accessibility documentation for the avatar component ([#973](https://github.com/Shopify/polaris-react/pull/973))
 - Updated docs about App Bridge usage in AppProvider ([#945](https://github.com/Shopify/polaris-react/pull/945))
 

src/components/AccountConnection/README.md

@@ -166,3 +166,13 @@ class AccountConnectionExample extends React.Component {
   };
 }
 ```
+
+---
+
+## Accessibility
+
+<!-- content-for: web -->
+
+See accessibility guidance for the [setting toggle component](/components/actions/setting-toggle) to turn connections on and off.
+
+<!-- /content-for-->

src/components/SettingToggle/README.md

@@ -115,3 +115,15 @@ class SettingToggleExample extends React.Component {
 ## Related components
 
 - To let merchants to connect or disconnect their store to third-party services and apps, [use the account connection component](/components/actions/account-connection)
+
+---
+
+## Accessibility
+
+<!-- content-for: web -->
+
+The setting toggle component is implemented as an HTML `<button>`. The current label should convey what happens when the button is pressed.
+
+To learn more about button accessibility, see the [button component](/components/actions/button).
+
+<!-- /content-for-->