docs(ngRepeat): extend description of tracking and duplicates

Add a section to the documentation on how tracking between items and DOM
elements is done, and why duplicates are not allowed in the collection.

Describe how the default tracking behaviour can be substituted with track by.

Tweak the wording in the `track by` section to discuss “tracking expressions”
instead of “tracking functions”.

Closes #8153

Author: teropa

src/ng/directive/ngRepeat.js

@@ -48,6 +48,55 @@
  * or implement a `$watch` on the object yourself.
  *
  *
+ * # Tracking and Duplicates
+ *
+ * When the contents of the collection change, `ngRepeat` makes the corresponding changes to the DOM:
+ *
+ * * When an item is added, a new instance of the template is added to the DOM.
+ * * When an item is removed, its template instance is removed from the DOM.
+ * * When items are reordered, their respective templates are reordered in the DOM.
+ *
+ * By default, `ngRepeat` does not allow duplicate items in arrays. This is because when
+ * there are duplicates, it is not possible to maintain a one-to-one mapping between collection
+ * items and DOM elements.
+ *
+ * If you do need to repeat duplicate items, you can substitute the default tracking behavior
+ * with your own using the `track by` expression.
+ *
+ * For example, you may track items by the index of each item in the collection, using the
+ * special scope property `$index`:
+ * ```html
+ *    <div ng-repeat="n in [42, 42, 43, 43] track by $index">
+ *      {{n}}
+ *    </div>
+ * ```
+ *
+ * You may use arbitrary expressions in `track by`, including references to custom functions
+ * on the scope:
+ * ```html
+ *    <div ng-repeat="n in [42, 42, 43, 43] track by myTrackingFunction(n)">
+ *      {{n}}
+ *    </div>
+ * ```
+ *
+ * If you are working with objects that have an identifier property, you can track
+ * by the identifier instead of the whole object. Should you reload your data later, `ngRepeat`
+ * will not have to rebuild the DOM elements for items it has already rendered, even if the
+ * JavaScript objects in the collection have been substituted for new ones:
+ * ```html
+ *    <div ng-repeat="model in collection track by model.id">
+ *      {{model.name}}
+ *    </div>
+ * ```
+ *
+ * When no `track by` expression is provided, it is equivalent to tracking by the built-in
+ * `$id` function, which tracks items by their identity:
+ * ```html
+ *    <div ng-repeat="obj in collection track by $id(obj)">
+ *      {{obj.prop}}
+ *    </div>
+ * ```
+ *
  * # Special repeat start and end points
  * To repeat a series of elements instead of just one parent element, ngRepeat (as well as other ng directives) supports extending
  * the range of the repeater by defining explicit start and end points by using **ng-repeat-start** and **ng-repeat-end** respectively.
@@ -115,12 +164,12 @@
  *
  *     For example: `(name, age) in {'adam':10, 'amalie':12}`.
  *
- *   * `variable in expression track by tracking_expression` – You can also provide an optional tracking function
- *     which can be used to associate the objects in the collection with the DOM elements. If no tracking function
- *     is specified the ng-repeat associates elements by identity in the collection. It is an error to have
- *     more than one tracking function to resolve to the same key. (This would mean that two distinct objects are
- *     mapped to the same DOM element, which is not possible.)  Filters should be applied to the expression,
- *     before specifying a tracking expression.
+ *   * `variable in expression track by tracking_expression` – You can also provide an optional tracking expression
+ *     which can be used to associate the objects in the collection with the DOM elements. If no tracking expression
+ *     is specified, ng-repeat associates elements by identity. It is an error to have
+ *     more than one tracking expression value resolve to the same key. (This would mean that two distinct objects are
+ *     mapped to the same DOM element, which is not possible.)  If filters are used in the expression, they should be
+ *     applied before the tracking expression.
  *
  *     For example: `item in items` is equivalent to `item in items track by $id(item)`. This implies that the DOM elements
  *     will be associated by item identity in the array.