docs(ngRepeat): document that track by must be the last expression

this is a point of confusion that's not well documented. see angular#5520

Closes angular#11934

Author: bcherny

src/ng/directive/ngRepeat.js

@@ -97,6 +97,15 @@
  *    </div>
  * ```
  *
+ * <div class="alert alert-warning">
+ * **Note:** `track by` must always be the last expression:
+ * </div>
+ * ```
+ * <div ng-repeat="model in collection | orderBy: 'id' as filtered_result track by model.id">
+ *     {{model.name}}
+ * </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.
@@ -168,8 +177,9 @@
  *     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.
+ *     mapped to the same DOM element, which is not possible.)
+ *
+ *     Note that the tracking expression must come last, after any filters, and the alias 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.