docs($rootScope): fix incorrect docs and make them clearer

During the first $digest loop after registering a $watch
the listener always run, so the example was incorrect

Closes #7598

Author: pomerantsev

src/ng/rootScope.js

@@ -261,13 +261,17 @@ function $RootScopeProvider(){
            expect(scope.counter).toEqual(0);
 
            scope.$digest();
-           // no variable change
-           expect(scope.counter).toEqual(0);
+           // the listener is always called during the first $digest loop after it was registered
+           expect(scope.counter).toEqual(1);
 
-           scope.name = 'adam';
            scope.$digest();
+           // but now it will not be called unless the value changes
            expect(scope.counter).toEqual(1);
 
+           scope.name = 'adam';
+           scope.$digest();
+           expect(scope.counter).toEqual(2);
+
 
 
            // Using a listener function
@@ -574,12 +578,16 @@ function $RootScopeProvider(){
            expect(scope.counter).toEqual(0);
 
            scope.$digest();
-           // no variable change
-           expect(scope.counter).toEqual(0);
+           // the listener is always called during the first $digest loop after it was registered
+           expect(scope.counter).toEqual(1);
 
-           scope.name = 'adam';
            scope.$digest();
+           // but now it will not be called unless the value changes
            expect(scope.counter).toEqual(1);
+
+           scope.name = 'adam';
+           scope.$digest();
+           expect(scope.counter).toEqual(2);
        * ```
        *
        */