Add a note about accessibilityRole compat

docs/guides/accessibility.md

@@ -39,7 +39,10 @@ using `aria-label`.
 
 In some cases, we also want to alert the end user of the type of selected
 component (i.e., that it is a “button”). To provide more context to screen
-readers, you should specify the `accessibilityRole` property.
+readers, you should specify the `accessibilityRole` property. (Note that React
+Native for Web provides a compatibility mapping of equivalent
+`accessibilityTraits` and `accessibilityComponentType` values to
+`accessibilityRole`).
 
 The `accessibilityRole` prop is used to infer an [analogous HTML
 element][html-aria-url] to use in addition to the resulting ARIA `role`, where

src/modules/AccessibilityUtil/propsToAriaRole.js

@@ -6,6 +6,7 @@ const accessibilityComponentTypeToRole = {
 const accessibilityTraitsToRole = {
   adjustable: 'slider',
   button: 'button',
+  header: 'heading',
   image: 'img',
   link: 'link',
   none: 'presentation',