Enable scaladoc and "classical" comments

It turns out that `/**` comments fail with the `-Xlint` option, so I just removed it for the `doc` task.
Also there is a weird literal inference with Scaladoc and the `Date` macro. My fix doesn't seem to affect the production code, just the Scaladoc production.
If we publish to a repository like Sonatype we'll have to provide the doc. Anyway this ci job is just about checking that the Scaladoc is working for now.