update api-async.mdx (#1163)

Author: lukaselmer

docs/dom-testing-library/api-async.mdx

@@ -45,7 +45,9 @@ function waitFor<T>(
 ```
 
 When in need to wait for any period of time you can use `waitFor`, to wait for
-your expectations to pass. Here's a simple example:
+your expectations to pass. Returning _a falsy condition is not sufficient_ to
+trigger a retry, the callback must throw an error in order to retry the
+condition. Here's a simple example:
 
 ```javascript
 // ...