imp: add the act type and hyperlink

Author: ssi02014

docs/react-testing-library/api.mdx

@@ -31,6 +31,7 @@ as these methods:
 - [`renderHook` Result](#renderhook-result)
   - [`result`](#result)
   - [`rerender`](#rerender-1)
+  - [`unmount`](#unmount-1)
 
 ---
 
@@ -324,6 +325,12 @@ All it does is forward all arguments to the act function if your version of
 react supports `act`. It is recommended to use the import from
 `@testing-library/react` over `react-dom/test-utils` for consistency reasons.
 
+```typescript
+export const act: typeof reactAct extends undefined
+  ? (callback: () => void) => void
+  : typeof reactAct
+```
+
 ## `renderHook`
 
 This is a convenience wrapper around `render` with a custom test component. The