[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.

project/SparkBuild.scala

@@ -723,8 +723,7 @@ object Unidoc {
         .map(_.filterNot(_.getCanonicalPath.contains("org/apache/hadoop")))
     },
 
-    // Javadoc options: create a window title, and group key packages on index page
-    javacOptions in doc := Seq(
+    javacOptions in (JavaUnidoc, unidoc) := Seq(
       "-windowtitle", "Spark " + version.value.replaceAll("-SNAPSHOT", "") + " JavaDoc",
       "-public",
       "-noqualifier", "java.lang"