Fix rescue_from documentation

I'm not sure whether this is a bug or intentional, but right now rescue_from **does not** rescue subclasses by default.

This seems to be caused by https://github.com/intridea/grape/blob/master/lib/grape/api.rb#L217

    handler_type = !!options[:rescue_subclasses] ? :rescue_handlers : :base_only_rescue_handlers