Add aria-activedescendant-has-tabindex rule in accessibility docs

site/content/docs/06-accessibility-warnings.md

@@ -19,6 +19,18 @@ Enforce no `accesskey` on element. Access keys are HTML attributes that allow we
 
 ---
 
+### `a11y-aria-activedescendant-has-tabindex`
+
+An element with `aria-activedescendant` must be tabbable, so it must either have an inherent `tabindex` or declare `tabindex` as an attribute.
+
+```sv
+<!-- A11y: Elements with attribute aria-activedescendant should have tabindex value -->
+<div aria-activedescendant="some-id" />
+
+```
+
+---
+
 ### `a11y-aria-attributes`
 
 Certain reserved DOM elements do not support ARIA roles, states and properties. This is often because they are not visible, for example `meta`, `html`, `script`, `style`. This rule enforces that these DOM elements do not contain the `aria-*` props.