docs: mention about optional done argument in test fn (jestjs#4556)

* docs(testAPI): mention about optional `done` argument

I've run into an issue that my test hangs because I've defined the parameter for the test callback. Could be handy to mention that.

* Update GlobalAPI.md

docs/en/GlobalAPI.md

@@ -288,7 +288,8 @@ test('did not rain', () => {
 
 The first argument is the test name; the second argument is a function that contains the expectations to test.
 
-If a promise is returned from `test`, Jest will wait for the promise to resolve before letting the test complete.
+> Note: If a **promise is returned** from `test`, Jest will wait for the promise to resolve before letting the test complete.
+Jest will also wait if you **provide an argument to the test function**, usually called `done`. This could be handy when you want to test callbacks. See how to test async code [here](http://facebook.github.io/jest/docs/en/asynchronous.html#callbacks).
 
 For example, let's say `fetchBeverageList()` returns a promise that is supposed to resolve to a list that has `lemon` in it. You can test this with: