In 1.4 and earlier releases, we have package grouping in the generated Java API docs. See http://spark.apache.org/docs/1.4.0/api/java/index.html. However, this disappeared in 1.5.0: http://spark.apache.org/docs/1.5.0/api/java/index.html.
Rather than fixing it, I'd suggest removing grouping. Because it might take some time to fix and it is a manual process to update the grouping in SparkBuild.scala. No one complained about missing groups since 1.5.0.