docs(ngModel): add section explaining that ngModel watches by reference

The new section explains that changing only a property on an object doesn't
trigger re-rendering.

Closes #13224
Closes #13518

Author: Thesoro

src/ng/directive/ngModel.js

@@ -921,6 +921,22 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
  *  - {@link ng.directive:select select}
  *  - {@link ng.directive:textarea textarea}
  *
+ * # Complex Models (objects or collections)
+ *
+ * By default, `ngModel` watches the model by reference, not value. This is important to know when
+ * binding inputs to models that are objects (e.g. `Date`) or collections (e.g. arrays). If only properties of the
+ * object or collection change, `ngModel` will not be notified and so the input will not be  re-rendered.
+ *
+ * The model must be assigned an entirely new object or collection before a re-rendering will occur.
+ *
+ * Some directives have options that will cause them to use a custom `$watchCollection` on the model expression
+ * - for example, `ngOptions` will do so when a `track by` clause is included in the comprehension expression or
+ * if the select is given the `multiple` attribute.
+ *
+ * The `$watchCollection()` method only does a shallow comparison, meaning that changing properties deeper than the
+ * first level of the object (or only changing the properties of an item in the collection if it's an array) will still
+ * not trigger a re-rendering of the model.
+ *
  * # CSS classes
  * The following CSS classes are added and removed on the associated input/select/textarea element
  * depending on the validity of the model.