Emphasise required return

Emphasise that `.rejects` and `.resolves` still require the assertion to be returned, in response to jestjs#4997.

docs/ExpectAPI.md

@@ -395,6 +395,8 @@ test('resolves to lemon', () => {
 });
 ```
 
+Note that, since you are still testing promises, the test is still asynchronous. Hence, you will need to [tell Jest to wait](TestingAsyncCode.md#promises) by returning the unwrapped assertion.
+
 Alternatively, you can use `async/await` in combination with `.resolves`:
 
 ```js
@@ -422,6 +424,8 @@ test('rejects to octopus', () => {
 });
 ```
 
+Note that, since you are still testing promises, the test is still asynchronous. Hence, you will need to [tell Jest to wait](TestingAsyncCode.md#promises) by returning the unwrapped assertion.
+
 Alternatively, you can use `async/await` in combination with `.rejects`.
 
 ```js