make scaladoc handle linking to external javadoc or scaladoc properly

Could not get the scaladoc parameter `-doc-external-doc` to work properly. As of creating this issue, this affects [PaxosSupervisor](../blob/master/src/main/scala/com/comcast/xfinity/sirius/api/impl/paxos/PaxosSupervisor.scala#L31) and [SiriusConfiguration](../blob/master/src/main/scala/com/comcast/xfinity/sirius/api/SiriusConfiguration.scala#L41). To see the linking warnings, run `sbt doc` at the root of the project.