docs(ngModel): document that objects passed to $setViewValue should be copied

[petebacondarwin]

ngModel doesn't watch $viewValue objects deeply and also does not make copies of those objects. This means that changes to the properties of those objects does not trigger ngModel changes.

So if your input directive provides objects (e.g. Date objects) to $setViewValue then it should make a copy before sending it into $setViewValue.

See #7965

[petebacondarwin]

@shahata - fancy knocking this up?