[docs] add docs for a11y-no-redundant-roles (sveltejs#7195)

site/content/docs/05-accessibility-warnings.md

@@ -213,6 +213,20 @@ Enforce that `on:mouseover` and `on:mouseout` are accompanied by `on:focus` and
 
 ---
 
+### `a11y-no-redundant-roles`
+
+Some HTML elements have default ARIA roles. Giving these elements an ARIA role that is already set by the browser [has no effect](https://www.w3.org/TR/using-aria/#aria-does-nothing) and is redundant.
+
+```sv
+<!-- A11y: Redundant role 'button' -->
+<button role="button" />
+
+<!-- A11y: Redundant role 'img' -->
+<img role="img" src="foo.jpg" />
+```
+
+---
+
 ### `a11y-positive-tabindex`
 
 Avoid positive `tabindex` property values. This will move elements out of the expected tab order, creating a confusing experience for keyboard users.