Explain where act comes from (#3237)

Co-authored-by: Sebastian Silbermann <[email protected]>

Author: koba04

content/docs/testing-recipes.md

@@ -57,7 +57,7 @@ You may use a different pattern, but keep in mind that we want to execute the cl
 
 ### `act()` {#act}
 
-When writing UI tests, tasks like rendering, user events, or data fetching can be considered as "units" of interaction with a user interface. React provides a helper called `act()` that makes sure all updates related to these "units" have been processed and applied to the DOM before you make any assertions:
+When writing UI tests, tasks like rendering, user events, or data fetching can be considered as "units" of interaction with a user interface. `react-dom/test-utils` provides a helper called [`act()`](/docs/test-utils.html#act) that makes sure all updates related to these "units" have been processed and applied to the DOM before you make any assertions:
 
 ```js
 act(() => {