[SPARK-18692][BUILD][DOCS] Test Java 8 unidoc build on Jenkins

## What changes were proposed in this pull request?

This PR proposes to run Spark unidoc to test Javadoc 8 build as Javadoc 8 is easily re-breakable.

There are several problems with it:

- It introduces little extra bit of time to run the tests. In my case, it took 1.5 mins more (`Elapsed :[94.8746569157]`). How it was tested is described in "How was this patch tested?".

- > One problem that I noticed was that Unidoc appeared to be processing test sources: if we can find a way to exclude those from being processed in the first place then that might significantly speed things up.

  (see  joshrosen's [comment](https://issues.apache.org/jira/browse/SPARK-18692?focusedCommentId=15947627&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15947627))

To complete this automated build, It also suggests to fix existing Javadoc breaks / ones introduced by test codes as described above.

There fixes are similar instances that previously fixed. Please refer https://github.com/apache/spark/pull/15999 and https://github.com/apache/spark/pull/16013

Note that this only fixes **errors** not **warnings**. Please see my observation https://github.com/apache/spark/pull/17389#issuecomment-288438704 for spurious errors by warnings.

## How was this patch tested?

Manually via `jekyll build` for building tests. Also, tested via running `./dev/run-tests`.

This was tested via manually adding `time.time()` as below:

```diff
     profiles_and_goals = build_profiles + sbt_goals

     print("[info] Building Spark unidoc (w/Hive 1.2.1) using SBT with these arguments: ",
           " ".join(profiles_and_goals))

+    import time
+    st = time.time()
     exec_sbt(profiles_and_goals)
+    print("Elapsed :[%s]" % str(time.time() - st))
```

produces

```
...
========================================================================
Building Unidoc API Documentation
========================================================================
...
[info] Main Java API documentation successful.
...
Elapsed :[94.8746569157]
...

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17477 from HyukjinKwon/SPARK-18692.

6 files changed, 26 insertions, 18 deletions

diff --git a/sql/core/src/test/scala/org/apache/spark/sql/DatasetSerializerRegistratorSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/DatasetSerializerRegistratorSuite.scala
index 0f3d0cefe3..92c5656f65 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/DatasetSerializerRegistratorSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/DatasetSerializerRegistratorSuite.scala
@@ -56,7 +56,9 @@ object TestRegistrator {
   def apply(): TestRegistrator = new TestRegistrator()
 }
 
-/** A [[Serializer]] that takes a [[KryoData]] and serializes it as KryoData(0). */
+/**
+ * A `Serializer` that takes a [[KryoData]] and serializes it as KryoData(0).
+ */
 class ZeroKryoDataSerializer extends Serializer[KryoData] {
   override def write(kryo: Kryo, output: Output, t: KryoData): Unit = {
     output.writeInt(0)
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSourceSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSourceSuite.scala
index 26967782f7..2108b118bf 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSourceSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/streaming/FileStreamSourceSuite.scala
@@ -44,8 +44,8 @@ abstract class FileStreamSourceTest
   import testImplicits._
 
   /**
-   * A subclass [[AddData]] for adding data to files. This is meant to use the
-   * [[FileStreamSource]] actually being used in the execution.
+   * A subclass `AddData` for adding data to files. This is meant to use the
+   * `FileStreamSource` actually being used in the execution.
    */
   abstract class AddFileData extends AddData {
     override def addData(query: Option[StreamExecution]): (Source, Offset) = {
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamSuite.scala
index 5ab9dc2bc7..13fe51a557 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamSuite.scala
@@ -569,7 +569,7 @@ class ThrowingIOExceptionLikeHadoop12074 extends FakeSource {
 
 object ThrowingIOExceptionLikeHadoop12074 {
   /**
-   * A latch to allow the user to wait until [[ThrowingIOExceptionLikeHadoop12074.createSource]] is
+   * A latch to allow the user to wait until `ThrowingIOExceptionLikeHadoop12074.createSource` is
    * called.
    */
   @volatile var createSourceLatch: CountDownLatch = null
@@ -600,7 +600,7 @@ class ThrowingInterruptedIOException extends FakeSource {
 
 object ThrowingInterruptedIOException {
   /**
-   * A latch to allow the user to wait until [[ThrowingInterruptedIOException.createSource]] is
+   * A latch to allow the user to wait until `ThrowingInterruptedIOException.createSource` is
    * called.
    */
   @volatile var createSourceLatch: CountDownLatch = null
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamingQuerySuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamingQuerySuite.scala
index 2ebbfcd22b..b69536ed37 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamingQuerySuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamingQuerySuite.scala
@@ -642,8 +642,10 @@ class StreamingQuerySuite extends StreamTest with BeforeAndAfter with Logging wi
    *
    * @param expectedBehavior  Expected behavior (not blocked, blocked, or exception thrown)
    * @param timeoutMs         Timeout in milliseconds
-   *                          When timeoutMs <= 0, awaitTermination() is tested (i.e. w/o timeout)
-   *                          When timeoutMs > 0, awaitTermination(timeoutMs) is tested
+   *                          When timeoutMs is less than or equal to 0, awaitTermination() is
+   *                          tested (i.e. w/o timeout)
+   *                          When timeoutMs is greater than 0, awaitTermination(timeoutMs) is
+   *                          tested
    * @param expectedReturnValue Expected return value when awaitTermination(timeoutMs) is used
    */
   case class TestAwaitTermination(
@@ -667,8 +669,10 @@ class StreamingQuerySuite extends StreamTest with BeforeAndAfter with Logging wi
      *
      * @param expectedBehavior  Expected behavior (not blocked, blocked, or exception thrown)
      * @param timeoutMs         Timeout in milliseconds
-     *                          When timeoutMs <= 0, awaitTermination() is tested (i.e. w/o timeout)
-     *                          When timeoutMs > 0, awaitTermination(timeoutMs) is tested
+     *                          When timeoutMs is less than or equal to 0, awaitTermination() is
+     *                          tested (i.e. w/o timeout)
+     *                          When timeoutMs is greater than 0, awaitTermination(timeoutMs) is
+     *                          tested
      * @param expectedReturnValue Expected return value when awaitTermination(timeoutMs) is used
      */
     def assertOnQueryCondition(
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/test/SQLTestUtils.scala b/sql/core/src/test/scala/org/apache/spark/sql/test/SQLTestUtils.scala
index cab219216d..6a4cc95d36 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/test/SQLTestUtils.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/test/SQLTestUtils.scala
@@ -41,11 +41,11 @@ import org.apache.spark.util.{UninterruptibleThread, Utils}
 /**
  * Helper trait that should be extended by all SQL test suites.
  *
- * This allows subclasses to plugin a custom [[SQLContext]]. It comes with test data
+ * This allows subclasses to plugin a custom `SQLContext`. It comes with test data
  * prepared in advance as well as all implicit conversions used extensively by dataframes.
- * To use implicit methods, import `testImplicits._` instead of through the [[SQLContext]].
+ * To use implicit methods, import `testImplicits._` instead of through the `SQLContext`.
  *
- * Subclasses should *not* create [[SQLContext]]s in the test suite constructor, which is
+ * Subclasses should *not* create `SQLContext`s in the test suite constructor, which is
  * prone to leaving multiple overlapping [[org.apache.spark.SparkContext]]s in the same JVM.
  */
 private[sql] trait SQLTestUtils
@@ -65,7 +65,7 @@ private[sql] trait SQLTestUtils
    * A helper object for importing SQL implicits.
    *
    * Note that the alternative of importing `spark.implicits._` is not possible here.
-   * This is because we create the [[SQLContext]] immediately before the first test is run,
+   * This is because we create the `SQLContext` immediately before the first test is run,
    * but the implicits import is needed in the constructor.
    */
   protected object testImplicits extends SQLImplicits {
@@ -73,7 +73,7 @@ private[sql] trait SQLTestUtils
   }
 
   /**
-   * Materialize the test data immediately after the [[SQLContext]] is set up.
+   * Materialize the test data immediately after the `SQLContext` is set up.
    * This is necessary if the data is accessed by name but not through direct reference.
    */
   protected def setupTestData(): Unit = {
@@ -250,8 +250,8 @@ private[sql] trait SQLTestUtils
   }
 
   /**
-   * Turn a logical plan into a [[DataFrame]]. This should be removed once we have an easier
-   * way to construct [[DataFrame]] directly out of local data without relying on implicits.
+   * Turn a logical plan into a `DataFrame`. This should be removed once we have an easier
+   * way to construct `DataFrame` directly out of local data without relying on implicits.
    */
   protected implicit def logicalPlanToSparkQuery(plan: LogicalPlan): DataFrame = {
     Dataset.ofRows(spark, plan)
@@ -271,7 +271,9 @@ private[sql] trait SQLTestUtils
     }
   }
 
-  /** Run a test on a separate [[UninterruptibleThread]]. */
+  /**
+   * Run a test on a separate `UninterruptibleThread`.
+   */
   protected def testWithUninterruptibleThread(name: String, quietly: Boolean = false)
     (body: => Unit): Unit = {
     val timeoutMillis = 10000
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/test/TestSQLContext.scala b/sql/core/src/test/scala/org/apache/spark/sql/test/TestSQLContext.scala
index b01977a238..959edf9a49 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/test/TestSQLContext.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/test/TestSQLContext.scala
@@ -22,7 +22,7 @@ import org.apache.spark.sql.SparkSession
 import org.apache.spark.sql.internal.{SessionState, SessionStateBuilder, SQLConf, WithTestConf}
 
 /**
- * A special [[SparkSession]] prepared for testing.
+ * A special `SparkSession` prepared for testing.
  */
 private[sql] class TestSparkSession(sc: SparkContext) extends SparkSession(sc) { self =>
   def this(sparkConf: SparkConf) {