Add documentation for Frame/Toast/Modal a11y improvements

polaris.shopify.com/content/components/feedback-indicators/toast.md

@@ -132,14 +132,16 @@ Action should:
 
 ## Accessibility
 
-The content of the toast component is implemented as an ARIA live region using `aria-live="polite"`. When the toast appears, screen readers should announce the toast text after any other more pressing announcements.
+The content of the toast component is implemented as an ARIA live region using `aria-live="assertive"`. When the toast appears, screen readers will interrupt any announcement a screen reader is currently making.
 
 Avoid using toast for critical information that merchants need to act on immediately. Toast might be difficult for merchants with low vision or low dexterity to access because it:
 
 - Disappears automatically
 - Can’t be easily accessed with the keyboard
 - Might appear outside the proximity of the merchant’s current focus
 
+To ensure that Toasts are read out by a screen reader when a Modal is open, apps should be wrapped in a [Frame](https://polaris.shopify.com/components/utilities/frame) component.
+
 ### Toast with action
 
 Make sure that merchants can also accomplish the action in the toast another way, since the toast action may be difficult to access for some merchants. If the toast action is not available somewhere else on the page, for example a retry action that reloads a section, it should have a fallback action, for example a browser refresh.

polaris.shopify.com/content/components/overlays/modal.md

@@ -246,6 +246,7 @@ Body content should be:
 - Modals use ARIA `role=”dialog”` to convey to screen reader users that they work like native dialog windows.
 - If you set the `title` prop to give the modal component a heading, then the `title` is used to label the dialog element with `aria-labelledby`. This helps to convey the purpose of the modal to screen reader users when it displays.
 - After a modal is closed, in order to return focus to the button that launched it, pass the button to the modal as an `activator`.
+- To ensure that [Toast](https://polaris.shopify.com/components/feedback-indicators/toast)s are read out by a screen reader when a Modal is open, apps should be wrapped in a [Frame](https://polaris.shopify.com/components/utilities/frame) component.
 
 ### Keyboard support