SPARK-3359 [DOCS] sbt/sbt unidoc doesn't work with Java 8

[srowen]

This follows #2893 , but does not completely fix SPARK-3359 either. This fixes minor scaladoc/javadoc issues that Javadoc 8 will treat as errors.

[SparkQA]

QA tests have started for PR 2909 at commit f62c347.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2909 at commit f62c347.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

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[mengxr]

@srowen Do you plan to fix more unidoc errors in this PR?

[srowen]

@mengxr This is all I think is fixable without starting to take away from the valid scaladoc. For example we'd have to remove @group tags and remove several links to methods. Really genjavadoc needs a few enhancements to address this otherwise. I think this is a step forward for the JIRA but does not fix it.

[mengxr]

LGTM. Verified that < shows up correctly in generated Scala and Java docs. Merged into master. Thanks!