docs(tutorial): add comment about injection annotation

Closes: #1163

docs/content/tutorial/step_05.ngdoc

@@ -138,6 +138,9 @@ describe('PhoneCat controllers', function() {
   describe('PhoneListCtrl', function(){
     var scope, ctrl, $httpBackend;
 
+    // The injector ignores leading and trailing underscores here (i.e. _$httpBackend_).
+    // This allows us to inject a service but then attach it to a variable
+    // with the same name as the service.
     beforeEach(inject(function(_$httpBackend_, $rootScope, $controller) {
       $httpBackend = _$httpBackend_;
       $httpBackend.expectGET('phones/phones.json').