docs(ngModel): further mods to $viewValue semantics

Closes #8110

Author: petebacondarwin

src/ng/directive/input.js

@@ -1577,9 +1577,19 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
    *
    * @description
    * Called when the view needs to be updated. It is expected that the user of the ng-model
-   * directive will implement this method. Note that in case `$modelValue` is an object,
-   * this method will be invoked only if a different instance is assigned to the model since
-   * `ngModel` does not perform a deep watch of objects.
+   * directive will implement this method.
+   *
+   * The `$render()` method is invoked in the following situations:
+   *
+   * * `$rollbackViewValue()` is called.  If we are rolling back the view value to the last
+   *   committed value then `$render()` is called to update the input control.
+   * * The value referenced by `ng-model` is changed programmatically and both the `$modelValue` and
+   *   the `$viewValue` are different to last time.
+   *
+   * Since `ng-model` does not do a deep watch, `$render()` is only invoked if the values of
+   * `$modelValue` and `$viewValue` are actually different to their previous value. If `$modelValue`
+   * or `$viewValue` are objects (rather than a string or number) then `$render()` will not be
+   * invoked if you only change a property on the objects.
    */
   this.$render = noop;
 
@@ -1880,15 +1890,25 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
    * @description
    * Update the view value.
    *
-   * This method should be called when the view value changes, typically from within a DOM event handler.
-   * For example {@link ng.directive:input input} and {@link ng.directive:select select}
-   * directives call it. In case the view value is an object, this method should be called with a
-   * copy of that object since `ngModel` does not perform a deep watch of objects. Properties of
-   * that copy should not be changed after `$setViewValue` in invoked.
+   * This method should be called when an input directive want to change the view value; typically,
+   * this is done from within a DOM event handler.
+   *
+   * For example {@link ng.directive:input input} calls it when the value of the input changes and
+   * {@link ng.directive:select select} calls it when an option is selected.
+   *
+   * If the new `value` is an object (rather than a string or a number), we should make a copy of the
+   * object before passing it to `$setViewValue`.  This is because `ngModel` does not perform a deep
+   * watch of objects, it only looks for a change of identity. If you only change the property of
+   * the object then ngModel will not realise that the object has changed and will not invoke the
+   * `$parsers` and `$validators` pipelines.
+   *
+   * For this reason, you should not change properties of the copy once it has been passed to
+   * `$setViewValue`. Otherwise you may cause the model value on the scope to change incorrectly.
    *
-   * It will update the $viewValue, then pass this value through each of the functions in `$parsers`,
-   * which includes any validators. The value that comes out of this `$parsers` pipeline, be applied to
-   * `$modelValue` and the **expression** specified in the `ng-model` attribute.
+   * When this method is called, the new `value` will be staged for committing through the `$parsers`
+   * and `$validators` pipelines. If there are no special {@link ngModelOptions} specified then the staged
+   * value sent directly for processing, finally to be applied to `$modelValue` and then the
+   * **expression** specified in the `ng-model` attribute.
    *
    * Lastly, all the registered change listeners, in the `$viewChangeListeners` list, are called.
    *