sbt 0.13 will be able to automatically configure scaladoc to link to external sites (via #191) for managed dependencies if those dependencies have a particular property in their pom.xml. This property name is info.apiURL and its value should be the base URL for the API docs for that module version.
So, the pom.xml for scala-library should have a section like:
This would be good to have for 2.10.2 so when sbt 0.13.0 is out, projects can auto-link their API docs to the Scala library's API docs. It will also help to verify that the feature works on the library every Scala project uses.