docs(guide/Unit Testing): add info about promises

Related #1915

Author: Narretz

docs/content/guide/unit-testing.ngdoc

@@ -430,5 +430,13 @@ If your directive uses `templateUrl`, consider using
 to pre-compile HTML templates and thus avoid having to load them over HTTP during test execution.
 Otherwise you may run into issues if the test directory hierarchy differs from the application's.
 
+## Testing Promises
+
+When testing promises, it's important to know that the resolution of promises is tied to the {@link ng.$rootScope.Scope#$digest digest cycle}.
+That means a promise's `then`, `catch` and `finally` callback functions are only called after a digest has run.
+In tests, you can trigger a digest by calling a scope's {@link ng.$rootScope.Scope#$apply `$apply` function}.
+If you don't have a scope in your test, you can inject the {@link ng.$rootScope $rootScope} and call `$apply` on it.
+There is also an example of testing promises in the {@link ng.$q#testing `$q` service documentation}.
+
 ## Sample project
 See the [angular-seed](https://github.com/angular/angular-seed) project for an example.

src/ng/q.js

@@ -186,7 +186,7 @@
  * - Q has many more features than $q, but that comes at a cost of bytes. $q is tiny, but contains
  *   all the important functionality needed for common async tasks.
  *
- *  # Testing
+ * # Testing
  *
  *  ```js
  *    it('should simulate promise', inject(function($q, $rootScope) {