The poms that relate to Scala have the maven-scala-plugin, which will generate Scaladocs if you navigate to the scala subproject and run
They will appear in target/site/scaladocs for the subproject.
This is not supported from the parent mahout pom
This should be incorporated into the process that publishes the javadocs. It appears the Scala code is not publically browsable as Scaladocs. Not sure where this process lives but the scala stuff should probably follow that same pattern/process as javadocs