docs(Scope): clarify naming in the $watch example

Fixes #8254
Closes #8271

Author: pkozlowski-opensource

src/ng/rootScope.js

@@ -248,7 +248,6 @@ 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
@@ -278,14 +277,14 @@ function $RootScopeProvider(){
 
 
 
-           // Using a listener function
+           // Using a function as a watchExpression
            var food;
            scope.foodCounter = 0;
            expect(scope.foodCounter).toEqual(0);
            scope.$watch(
-             // This is the listener function
+             // This function returns the value being watched. It is called for each turn of the $digest loop
              function() { return food; },
-             // This is the change handler
+             // This is the change listener, called when the value returned from the above function changes
              function(newValue, oldValue) {
                if ( newValue !== oldValue ) {
                  // Only increment the counter if the value changed