docs(ngOptions): add info about preselecting complex models

Closes angular#12966

Author: Narretz

src/ng/directive/ngOptions.js

@@ -33,20 +33,28 @@ var ngOptionsMinErr = minErr('ngOptions');
  *
  * ## Complex Models (objects or collections)
  *
- * **Note:** By default, `ngModel` watches the model by reference, not value. This is important when
- * binding any input directive to a model that is an object or a collection.
+ * By default, `ngModel` watches the model by reference, not value. This is important to know when
+ * binding the select to a model that is an object or a collection.
  *
- * Since this is a common situation for `ngOptions` the directive additionally watches the model using
- * `$watchCollection` when the select has the `multiple` attribute or when there is a `track by` clause in
- * the options expression. This allows ngOptions to trigger a re-rendering of the options even if the actual
- * object/collection has not changed identity but only a property on the object or an item in the collection
- * changes.
+ * One issue occurs if you want to preselect an option. For example, if you set
+ * the model to an object that is equal to an object in your collection, `ngOptions` won't be able to set the selection,
+ * because the objects are not identical. So by default, you should always reference the item in your collection
+ * for preselections, e.g.: `$scope.selected = $scope.collection[3]`.
+ *
+ * Another solution is to use a `track by` clause, because then `ngOptions` will track the identity
+ * of the item not by reference, but by the result of the `track by` expression. For example, if your
+ * collection items have an id property, you would `track by item.id`.
+ *
+ * A different issue with objects or collections is that ngModel won't detect if an object property or
+ * a collection item changes. For that reason, `ngOptions` additionally watches the model using
+ * `$watchCollection`, when the expression contains a `track by` clause or the the select has the `multiple` attribute.
+ * This allows ngOptions to trigger a re-rendering of the options even if the actual object/collection
+ * has not changed identity, but only a property on the object or an item in the collection changes.
  *
  * Note that `$watchCollection` does a shallow comparison of the properties of the object (or the items in the collection
  * if the model is an array). This means that changing a property deeper than the first level inside the
  * object/collection will not trigger a re-rendering.
  *
- *
  * ## `select` **`as`**
  *
  * Using `select` **`as`** will bind the result of the `select` expression to the model, but