Add note on broken react runtime types

Closes GH-2383.

Reviewed-by: Titus Wormer <[email protected]>

Author: ky1ejs

docs/migrating/v3.mdx

@@ -52,6 +52,20 @@ You will get a runtime error if these features are used in MDX without
 If you passed the `useDynamicImport` option before, remove it, the behavior
 is now the default.
 
+If you use `react/jsx-runtime`, you might get a TypeScript error (such as
+`Property 'Fragment' is missing in type`), because it is typed incorrectly.
+To remediate this, do:
+
+```tsx
+import {type Fragment, type Jsx, run} from '@mdx-js/mdx'
+import * as runtime_ from 'react/jsx-runtime'
+
+// @ts-expect-error: the automatic react runtime is untyped.
+const runtime: {Fragment: Fragment; jsx: Jsx; jsxs: Jsx} = runtime_
+
+const result = await run('# hi', {...runtime, baseUrl: import.meta.url})
+```
+
 ## Use the automatic JSX runtime
 
 If you use the classic runtime, switch to the automatic runtime.