It seems that `(ScalaUnidoc, unidoc)` is the correct way to overwrite `scalacOptions` in unidoc.

CC: rxin gzm0

Author: Xiangrui Meng <meng@databricks.com>

Closes #4404 from mengxr/SPARK-5620 and squashes the following commits:

f890cf5 [Xiangrui Meng] add -groups to scalacOptions in unidoc