add more additional line comments to display-a-button-to-remove-completed-todos code sample

I could envision a noob user getting confused and replacing code within actions instead of appending code to actions within the code example I supplied a change for.

Below, I output the todos_controller.js before and after the code change demonstrated by the code sample I added extra 'additional lines' comments to

controller before code example change:
````javascript
Todos.TodosController = Ember.ArrayController.extend({
  actions: {
    createTodo: function () {
      // Get the todo title set by the "New Todo" text field
      var title = this.get('newTitle');
      if (!title.trim()) { return; }

      // Create the new Todo model
      var todo = this.store.createRecord('todo', {
        title: title,
        isCompleted: false
      });

      // Clear the "New Todo" text field
      this.set('newTitle', '');

      // Save the new model
      todo.save();
    }
  },

  remaining: function () {
    return this.filterProperty('isCompleted', false).get('length');
  }.property('@each.isCompleted'),

  inflection: function () {
    var remaining = this.get('remaining');
    return remaining === 1 ? 'item' : 'items';
  }.property('remaining')
});
````
controller following code example change:
````javascript
Todos.TodosController = Ember.ArrayController.extend({
  actions: {
    createTodo: function () {
      // Get the todo title set by the "New Todo" text field
      var title = this.get('newTitle');
      if (!title.trim()) { return; }

      // Create the new Todo model
      var todo = this.store.createRecord('todo', {
        title: title,
        isCompleted: false
      });

      // Clear the "New Todo" text field
      this.set('newTitle', '');

      // Save the new model
      todo.save();
    },
    clearCompleted: function () {
      var completed = this.filterProperty('isCompleted', true);
      completed.invoke('deleteRecord');
      completed.invoke('save');
    }
  },

  remaining: function () {
    return this.filterProperty('isCompleted', false).get('length');
  }.property('@each.isCompleted'),

  inflection: function () {
    var remaining = this.get('remaining');
    return remaining === 1 ? 'item' : 'items';
  }.property('remaining'),

  hasCompleted: function () {
    return this.get('completed') > 0;
  }.property('completed'),

  completed: function () {
    return this.filterProperty('isCompleted', true).get('length');
  }.property('@each.isCompleted')
});
````