docs: document Vitest SSR environment requirement for Astro 6

src/content/docs/en/guides/upgrade-to/v6.mdx

@@ -1307,6 +1307,30 @@ export function getStaticPaths() {
 
 <ReadMore>Learn more about [dynamic SSG routes with `getStaticPaths()`](/en/guides/routing/#static-ssg-mode).</ReadMore>
 
+### Changed: Astro components cannot be rendered in Vitest client environments (Container API)
+
+<SourcePR number="14895" title="feat: remove Vitest workaround for client environment"/>
+
+In Astro 5.x, rendering an Astro component on the client was forbidden. However we temporarily allowed this behavoir in Vitest client environments such as `jsdom` or `happy-dom` using the [experimental Container API](/en/reference/container-reference/).
+
+Astro 6.0 removes the ability to render Astro components in Vitest client environments: tests that render Astro components must now run in a server environment like `node`.
+
+#### What should I do? 
+
+If you use Vitest to run tests that render Astro components in client environments like `jsdom` or `happy-dom`, update your Vitest config to use the `node` environment for these:
+```ts title="vitest.config.ts" del={5} ins={6}
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'jsdom',
+    environment: 'node',
+  },
+});
+```
+
+<ReadMore>Learn more about [testing Astro components](/en/guides/testing/).</ReadMore>
+
 ### Changed: Integration hooks and HMR access patterns (Integration API)
 
 <SourcePR number="14306" title="feat: integrate vite environments"/>