Explain that default_scope also influences update_all

This was not explicitly stated before and I needed to try it out to be
certain. A little explicit statement in the API docs might help here.
The example above already cover this same thing. The difference is that your example is using implicit chaining with `default_scope`. I prefer to not show this case mainly because it give the wrong impression that `default_scope` is a special relation when it is just a way to configure a default relation.