Add back docs for the old way of testing promises (jestjs#3256)

* Add back docs for the old way of testing promises

Fixes jestjs#3250

* Add `return` note to old version

docs/TestingAsyncCode.md

@@ -48,12 +48,24 @@ If `done()` is never called, the test will fail, which is what you want to happe
 
 ### Promises
 
-##### available in Jest **20.0.0+**
-
-If your code uses promises, there is a simpler way to handle asynchronous tests. Just use the `resolves` keyword in your expect statement, and Jest will wait for that promise to resolve. If the promise is rejected, the test will automatically fail.
+If your code uses promises, there is a simpler way to handle asynchronous tests. Just return a promise from your test, and Jest will wait for that promise to resolve. If the promise is rejected, the test will automatically fail.
 
 For example, let's say that `fetchData`, instead of using a callback, returns a promise that is supposed to resolve to the string `"peanut butter"`. We could test it with:
 
+```js
+test('the data is peanut butter', () => {
+ return fetchData().then(data => {
+ expect(data).toBe('peanut butter');
+ });
+});
+```
+
+Be sure to return the promise - if you omit this `return` statement, your test will complete before `fetchData` completes.
+
+##### available in Jest **20.0.0+**
+
+You can also use the `resolves` keyword in your expect statement, and Jest will wait for that promise to resolve. If the promise is rejected, the test will automatically fail.
+
 ```js
 test('the data is peanut butter', () => {
  return expect(fetchData()).resolves.toBe('peanut butter');