docs(ngRepeat): add more info about watching and tracking

- mention $watchCollection
- highlight that track by "id" can improve render performance

Related #9508

Author: Narretz

src/ng/directive/ngRepeat.js

@@ -53,15 +53,21 @@
  *
  * # Tracking and Duplicates
  *
- * When the contents of the collection change, `ngRepeat` makes the corresponding changes to the DOM:
+ * `ngRepeat` uses {@link $rootScope.Scope#$watchCollection $watchCollection} to detect changes in
+ * the collection. When a change happens, ngRepeat then makes the corresponding changes to the DOM:
  *
  * * When an item is added, a new instance of the template is added to the DOM.
  * * When an item is removed, its template instance is removed from the DOM.
  * * When items are reordered, their respective templates are reordered in the DOM.
  *
- * By default, `ngRepeat` does not allow duplicate items in arrays. This is because when
- * there are duplicates, it is not possible to maintain a one-to-one mapping between collection
- * items and DOM elements.
+ * To minimize creation of DOM elements, `ngRepeat` uses a function
+ * to "keep track" of all items in the collection and their corresponding DOM elements.
+ * For example, if an item is added to the collection, ngRepeat will know that all other items
+ * already have DOM elements, and will not re-render them.
+ *
+ * The default tracking function (which tracks items by their identity) does not allow
+ * duplicate items in arrays. This is because when there are duplicates, it is not possible
+ * to maintain a one-to-one mapping between collection items and DOM elements.
  *
  * If you do need to repeat duplicate items, you can substitute the default tracking behavior
  * with your own using the `track by` expression.
@@ -74,18 +80,22 @@
  *    </div>
  * ```
  *
- * You may use arbitrary expressions in `track by`, including references to custom functions
+ * You may also use arbitrary expressions in `track by`, including references to custom functions
  * on the scope:
  * ```html
  *    <div ng-repeat="n in [42, 42, 43, 43] track by myTrackingFunction(n)">
  *      {{n}}
  *    </div>
  * ```
  *
- * If you are working with objects that have an identifier property, you can track
+ * <div class="alert alert-success">
+ * If you are working with objects that have an identifier property, you should track
  * by the identifier instead of the whole object. Should you reload your data later, `ngRepeat`
  * will not have to rebuild the DOM elements for items it has already rendered, even if the
- * JavaScript objects in the collection have been substituted for new ones:
+ * JavaScript objects in the collection have been substituted for new ones. For large collections,
+ * this signifincantly improves rendering performance. If you don't have a unique identifier,
+ * `track by $index` can also provide a performance boost.
+ * </div>
  * ```html
  *    <div ng-repeat="model in collection track by model.id">
  *      {{model.name}}