[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.

11 files changed, 21 insertions, 21 deletions

diff --git a/core/src/main/scala/org/apache/spark/rpc/RpcEndpoint.scala b/core/src/main/scala/org/apache/spark/rpc/RpcEndpoint.scala
index 0ba9516952..97eed540b8 100644
--- a/core/src/main/scala/org/apache/spark/rpc/RpcEndpoint.scala
+++ b/core/src/main/scala/org/apache/spark/rpc/RpcEndpoint.scala
@@ -35,7 +35,7 @@ private[spark] trait RpcEnvFactory {
  *
  * The life-cycle of an endpoint is:
  *
- * constructor -> onStart -> receive* -> onStop
+ * {@code constructor -> onStart -> receive* -> onStop}
  *
  * Note: `receive` can be called concurrently. If you want `receive` to be thread-safe, please use
  * [[ThreadSafeRpcEndpoint]]
@@ -63,16 +63,16 @@ private[spark] trait RpcEndpoint {
   }
 
   /**
-   * Process messages from [[RpcEndpointRef.send]] or [[RpcCallContext.reply)]]. If receiving a
-   * unmatched message, [[SparkException]] will be thrown and sent to `onError`.
+   * Process messages from `RpcEndpointRef.send` or `RpcCallContext.reply`. If receiving a
+   * unmatched message, `SparkException` will be thrown and sent to `onError`.
    */
   def receive: PartialFunction[Any, Unit] = {
     case _ => throw new SparkException(self + " does not implement 'receive'")
   }
 
   /**
-   * Process messages from [[RpcEndpointRef.ask]]. If receiving a unmatched message,
-   * [[SparkException]] will be thrown and sent to `onError`.
+   * Process messages from `RpcEndpointRef.ask`. If receiving a unmatched message,
+   * `SparkException` will be thrown and sent to `onError`.
    */
   def receiveAndReply(context: RpcCallContext): PartialFunction[Any, Unit] = {
     case _ => context.sendFailure(new SparkException(self + " won't reply anything"))
diff --git a/core/src/main/scala/org/apache/spark/rpc/RpcTimeout.scala b/core/src/main/scala/org/apache/spark/rpc/RpcTimeout.scala
index 2c9a976e76..0557b7a3cc 100644
--- a/core/src/main/scala/org/apache/spark/rpc/RpcTimeout.scala
+++ b/core/src/main/scala/org/apache/spark/rpc/RpcTimeout.scala
@@ -26,7 +26,7 @@ import org.apache.spark.SparkConf
 import org.apache.spark.util.{ThreadUtils, Utils}
 
 /**
- * An exception thrown if RpcTimeout modifies a [[TimeoutException]].
+ * An exception thrown if RpcTimeout modifies a `TimeoutException`.
  */
 private[rpc] class RpcTimeoutException(message: String, cause: TimeoutException)
   extends TimeoutException(message) { initCause(cause) }
diff --git a/core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala b/core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala
index 0971731683..aab177f257 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala
@@ -607,7 +607,7 @@ class DAGScheduler(
    * @param resultHandler callback to pass each result to
    * @param properties scheduler properties to attach to this job, e.g. fair scheduler pool name
    *
-   * @throws Exception when the job fails
+   * @note Throws `Exception` when the job fails
    */
   def runJob[T, U](
       rdd: RDD[T],
@@ -644,7 +644,7 @@ class DAGScheduler(
    *
    * @param rdd target RDD to run tasks on
    * @param func a function to run on each partition of the RDD
-   * @param evaluator [[ApproximateEvaluator]] to receive the partial results
+   * @param evaluator `ApproximateEvaluator` to receive the partial results
    * @param callSite where in the user program this job was called
    * @param timeout maximum time to wait for the job, in milliseconds
    * @param properties scheduler properties to attach to this job, e.g. fair scheduler pool name
diff --git a/core/src/main/scala/org/apache/spark/scheduler/ExternalClusterManager.scala b/core/src/main/scala/org/apache/spark/scheduler/ExternalClusterManager.scala
index d1ac7131ba..47f3527a32 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/ExternalClusterManager.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/ExternalClusterManager.scala
@@ -42,7 +42,7 @@ private[spark] trait ExternalClusterManager {
 
   /**
    * Create a scheduler backend for the given SparkContext and scheduler. This is
-   * called after task scheduler is created using [[ExternalClusterManager.createTaskScheduler()]].
+   * called after task scheduler is created using `ExternalClusterManager.createTaskScheduler()`.
    * @param sc SparkContext
    * @param masterURL the master URL
    * @param scheduler TaskScheduler that will be used with the scheduler backend.
diff --git a/core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala b/core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala
index c849a16023..1b6bc9139f 100644
--- a/core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala
+++ b/core/src/main/scala/org/apache/spark/scheduler/TaskSchedulerImpl.scala
@@ -38,7 +38,7 @@ import org.apache.spark.util.{AccumulatorV2, ThreadUtils, Utils}
 
 /**
  * Schedules tasks for multiple types of clusters by acting through a SchedulerBackend.
- * It can also work with a local setup by using a [[LocalSchedulerBackend]] and setting
+ * It can also work with a local setup by using a `LocalSchedulerBackend` and setting
  * isLocal to true. It handles common logic, like determining a scheduling order across jobs, waking
  * up to launch speculative tasks, etc.
  *
@@ -704,12 +704,12 @@ private[spark] object TaskSchedulerImpl {
    * Used to balance containers across hosts.
    *
    * Accepts a map of hosts to resource offers for that host, and returns a prioritized list of
-   * resource offers representing the order in which the offers should be used.  The resource
+   * resource offers representing the order in which the offers should be used. The resource
    * offers are ordered such that we'll allocate one container on each host before allocating a
    * second container on any host, and so on, in order to reduce the damage if a host fails.
    *
-   * For example, given <h1, [o1, o2, o3]>, <h2, [o4]>, <h1, [o5, o6]>, returns
-   * [o1, o5, o4, 02, o6, o3]
+   * For example, given {@literal <h1, [o1, o2, o3]>}, {@literal <h2, [o4]>} and
+   * {@literal <h3, [o5, o6]>}, returns {@literal [o1, o5, o4, o2, o6, o3]}.
    */
   def prioritizeContainers[K, T] (map: HashMap[K, ArrayBuffer[T]]): List[T] = {
     val _keyList = new ArrayBuffer[K](map.size)
diff --git a/core/src/main/scala/org/apache/spark/storage/BlockManager.scala b/core/src/main/scala/org/apache/spark/storage/BlockManager.scala
index 63acba65d3..3219969bcd 100644
--- a/core/src/main/scala/org/apache/spark/storage/BlockManager.scala
+++ b/core/src/main/scala/org/apache/spark/storage/BlockManager.scala
@@ -66,7 +66,7 @@ private[spark] trait BlockData {
   /**
    * Returns a Netty-friendly wrapper for the block's data.
    *
-   * @see [[ManagedBuffer#convertToNetty()]]
+   * Please see `ManagedBuffer.convertToNetty()` for more details.
    */
   def toNetty(): Object
 
diff --git a/core/src/test/scala/org/apache/spark/AccumulatorSuite.scala b/core/src/test/scala/org/apache/spark/AccumulatorSuite.scala
index 6d03ee091e..ddbcb2d19d 100644
--- a/core/src/test/scala/org/apache/spark/AccumulatorSuite.scala
+++ b/core/src/test/scala/org/apache/spark/AccumulatorSuite.scala
@@ -243,7 +243,7 @@ private[spark] object AccumulatorSuite {
   import InternalAccumulator._
 
   /**
-   * Create a long accumulator and register it to [[AccumulatorContext]].
+   * Create a long accumulator and register it to `AccumulatorContext`.
    */
   def createLongAccum(
       name: String,
@@ -258,7 +258,7 @@ private[spark] object AccumulatorSuite {
   }
 
   /**
-   * Make an [[AccumulableInfo]] out of an [[Accumulable]] with the intent to use the
+   * Make an `AccumulableInfo` out of an [[Accumulable]] with the intent to use the
    * info as an accumulator update.
    */
   def makeInfo(a: AccumulatorV2[_, _]): AccumulableInfo = a.toInfo(Some(a.value), None)
diff --git a/core/src/test/scala/org/apache/spark/ExternalShuffleServiceSuite.scala b/core/src/test/scala/org/apache/spark/ExternalShuffleServiceSuite.scala
index eb3fb99747..fe944031bc 100644
--- a/core/src/test/scala/org/apache/spark/ExternalShuffleServiceSuite.scala
+++ b/core/src/test/scala/org/apache/spark/ExternalShuffleServiceSuite.scala
@@ -27,7 +27,7 @@ import org.apache.spark.network.shuffle.{ExternalShuffleBlockHandler, ExternalSh
 /**
  * This suite creates an external shuffle server and routes all shuffle fetches through it.
  * Note that failures in this suite may arise due to changes in Spark that invalidate expectations
- * set up in [[ExternalShuffleBlockHandler]], such as changing the format of shuffle files or how
+ * set up in `ExternalShuffleBlockHandler`, such as changing the format of shuffle files or how
  * we hash files into folders.
  */
 class ExternalShuffleServiceSuite extends ShuffleSuite with BeforeAndAfterAll {
diff --git a/core/src/test/scala/org/apache/spark/LocalSparkContext.scala b/core/src/test/scala/org/apache/spark/LocalSparkContext.scala
index 24ec99c7e5..1dd89bcbe3 100644
--- a/core/src/test/scala/org/apache/spark/LocalSparkContext.scala
+++ b/core/src/test/scala/org/apache/spark/LocalSparkContext.scala
@@ -22,7 +22,7 @@ import org.scalatest.BeforeAndAfterAll
 import org.scalatest.BeforeAndAfterEach
 import org.scalatest.Suite
 
-/** Manages a local `sc` {@link SparkContext} variable, correctly stopping it after each test. */
+/** Manages a local `sc` `SparkContext` variable, correctly stopping it after each test. */
 trait LocalSparkContext extends BeforeAndAfterEach with BeforeAndAfterAll { self: Suite =>
 
   @transient var sc: SparkContext = _
diff --git a/core/src/test/scala/org/apache/spark/scheduler/SchedulerIntegrationSuite.scala b/core/src/test/scala/org/apache/spark/scheduler/SchedulerIntegrationSuite.scala
index 8103983c43..8300607ea8 100644
--- a/core/src/test/scala/org/apache/spark/scheduler/SchedulerIntegrationSuite.scala
+++ b/core/src/test/scala/org/apache/spark/scheduler/SchedulerIntegrationSuite.scala
@@ -95,12 +95,12 @@ abstract class SchedulerIntegrationSuite[T <: MockBackend: ClassTag] extends Spa
   }
 
   /**
-   * A map from partition -> results for all tasks of a job when you call this test framework's
+   * A map from partition to results for all tasks of a job when you call this test framework's
    * [[submit]] method.  Two important considerations:
    *
    * 1. If there is a job failure, results may or may not be empty.  If any tasks succeed before
    * the job has failed, they will get included in `results`.  Instead, check for job failure by
-   * checking [[failure]].  (Also see [[assertDataStructuresEmpty()]])
+   * checking [[failure]]. (Also see `assertDataStructuresEmpty()`)
    *
    * 2. This only gets cleared between tests.  So you'll need to do special handling if you submit
    * more than one job in one test.
diff --git a/core/src/test/scala/org/apache/spark/serializer/SerializerPropertiesSuite.scala b/core/src/test/scala/org/apache/spark/serializer/SerializerPropertiesSuite.scala
index 4ce3b941be..99882bf76e 100644
--- a/core/src/test/scala/org/apache/spark/serializer/SerializerPropertiesSuite.scala
+++ b/core/src/test/scala/org/apache/spark/serializer/SerializerPropertiesSuite.scala
@@ -29,7 +29,7 @@ import org.apache.spark.serializer.KryoTest.RegistratorWithoutAutoReset
 /**
  * Tests to ensure that [[Serializer]] implementations obey the API contracts for methods that
  * describe properties of the serialized stream, such as
- * [[Serializer.supportsRelocationOfSerializedObjects]].
+ * `Serializer.supportsRelocationOfSerializedObjects`.
  */
 class SerializerPropertiesSuite extends SparkFunSuite {