Add -Xfatal-warnings even for scaladoc generation

[oleastre]

For some sub-projects, scaladoc generation emit warnings and since #1912 fails.

As a temporary solution #1916 disables fatal warnings for doc compilation.

[gzm0]

The main problem here is links to external docs / java doc. They seem to be hard to generate.

Should we:

1. Not have links to JavaDoc (use backquotes instead of [abc] for links)
2. Try really hard and maybe get it to work

Opinions?

[sjrd]

We should try to make them work. Supposedly there's a semi automatic way to do that, assuming we can get the right sbt config.

[SethTisue]

gory details at https://issues.scala-lang.org/browse/SI-9311

[gzm0]

Thanks @SethTisue for the pointer. I will drop issues I encounter in here as I go.

[gzm0]

First two issues:

Linking to other Scala.js libraries

External documentation (via -doc-external-doc switch or sbt's apiMapping builtin) seems to be only linkable if the dependency is in a JAR (specifying a classpath does not work).

Therefore, exportJars needs to be set to true.

This is an annoyance but definitely not a complete show stopper.

Linking to external nested classes

The tools link to the member ir.Types.ClassType. The reference to it can be successfully resolved. However, the generated link is wrong: #org.scalajs.core.ir.Types$.ClassType instead of #org.scalajs.core.ir.Types$$ClassType.

This is probably nothing some relatively sophisticated regex processing can't solve, but it's cumbersome and hard to notice if it fails.