Usage of `@field` to export class constructor fields is wrong/obsolete

It seems that [this section](http://www.scala-js.org/doc/interoperability/export-to-javascript.html#-export-fields-directly-declared-in-constructors) is incorrect or has been obsoleted at some point in the past.

See [discussion](https://gitter.im/scala-js/scala-js?at=5ee36a7d24a3382d5d616fa4).

