In README.md under docs/ directory, it says that

> You can build just the Spark scaladoc by running build/sbt doc from the SPARK_PROJECT_ROOT directory.

I guess the right approach is build/sbt unidoc

Author: CodingCat <zhunansjtu@gmail.com>

Closes #5253 from CodingCat/SPARK-6596 and squashes the following commits:

af379ed [CodingCat] fix the instruction on building scaladoc