Add comment about test expectation

Author: willshowell

src/lib/dialog/dialog.spec.ts

@@ -213,6 +213,7 @@ describe('MdDialog', () => {
     viewContainerFixture.detectChanges();
 
     viewContainerFixture.whenStable().then(() => {
+      // Additional clicks after the dialog has closed should not be emitted
       backdrop.click();
       expect(spy).toHaveBeenCalledTimes(1);
     });