docs(rootScope): add example of using a listener function for $watch

Closes #4451

src/ng/rootScope.js

@@ -240,6 +240,8 @@ function $RootScopeProvider(){
        * can compare the `newVal` and `oldVal`. If these two values are identical (`===`) then the
        * listener was called due to initialization.
        *
+       * The example below contains an illustration of using a function as your $watch listener
+       *
        *
        * # Example
        * <pre>
@@ -261,6 +263,36 @@ function $RootScopeProvider(){
            scope.name = 'adam';
            scope.$digest();
            expect(scope.counter).toEqual(1);
+
+
+
+           // Using a listener function 
+           var food;
+           scope.foodCounter = 0;
+           expect(scope.foodCounter).toEqual(0);
+           scope.$watch(
+             // This is the listener function
+             function() { return food; },
+             // This is the change handler
+             function(newValue, oldValue) {
+               if ( newValue !== oldValue ) {
+                 // Only increment the counter if the value changed
+                 scope.foodCounter = scope.foodCounter + 1;
+               }
+             }
+           );
+           // No digest has been run so the counter will be zero
+           expect(scope.foodCounter).toEqual(0);
+
+           // Run the digest but since food has not changed cout will still be zero
+           scope.$digest();
+           expect(scope.foodCounter).toEqual(0);
+
+           // Update food and run digest.  Now the counter will increment
+           food = 'cheeseburger';
+           scope.$digest();
+           expect(scope.foodCounter).toEqual(1);  
+
        * </pre>
        *
        *