[SPARK-16353][BUILD][DOC] Missing javadoc options for java unidoc
Link to Jira issue: https://issues.apache.org/jira/browse/SPARK-16353
## What changes were proposed in this pull request?
The javadoc options for the java unidoc generation are ignored when generating the java unidoc. For example, the generated `index.html` has the wrong HTML page title. This can be seen at http://spark.apache.org/docs/latest/api/java/index.html.
I changed the relevant setting scope from `doc` to `(JavaUnidoc, unidoc)`.
## How was this patch tested?
I ran `docs/jekyll build` and verified that the java unidoc `index.html` has the correct HTML page title.
Author: Michael Allman <michael@videoamp.com>
Closes #14031 from mallman/spark-16353.
(cherry picked from commit 7dbffcdd6dc76b8e8d6a9cd6eeb24323a6b740c3)
Signed-off-by: Sean Owen <sowen@cloudera.com>
