Issues importing docs into Dash

This is a continuation of https://github.com/akka/akka/issues/17860.

Dash generates the docsets by downloading the javadoc.jar and indexing it.

While generating docsets for akka-streams one of the following issues occurs:

• The docs aren't there. Some of the packages just have a MANIFEST.MF file inside their javadoc.jar, and nothing else, so there's nothing to generate.
• The docs aren't generated using scaladoc, but instead with javadoc. For example, com.typesafe.akka:akka-stream-experimental_2.11 has the docs generated with javadoc. When something is installed from "Scala Docsets", it expects to find scaladoc not javadoc docs. As a workaround docsets for these packages can be installed from "Java Docsets".

I don't understand much about Akka/Scala and so on, so I'm most likely missing something, but why are you generating the docs using javadoc and not scaladoc? Is this something that is common for Akka and I should support it on my end?