[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.

3 files changed, 4 insertions, 4 deletions

diff --git a/streaming/src/main/scala/org/apache/spark/streaming/rdd/MapWithStateRDD.scala b/streaming/src/main/scala/org/apache/spark/streaming/rdd/MapWithStateRDD.scala
index 58b7031d5e..15d3c7e54b 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/rdd/MapWithStateRDD.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/rdd/MapWithStateRDD.scala
@@ -29,7 +29,7 @@ import org.apache.spark.streaming.util.{EmptyStateMap, StateMap}
 import org.apache.spark.util.Utils
 
 /**
- * Record storing the keyed-state [[MapWithStateRDD]]. Each record contains a [[StateMap]] and a
+ * Record storing the keyed-state [[MapWithStateRDD]]. Each record contains a `StateMap` and a
  * sequence of records returned by the mapping function of `mapWithState`.
  */
 private[streaming] case class MapWithStateRDDRecord[K, S, E](
@@ -111,7 +111,7 @@ private[streaming] class MapWithStateRDDPartition(
 /**
  * RDD storing the keyed states of `mapWithState` operation and corresponding mapped data.
  * Each partition of this RDD has a single record of type [[MapWithStateRDDRecord]]. This contains a
- * [[StateMap]] (containing the keyed-states) and the sequence of records returned by the mapping
+ * `StateMap` (containing the keyed-states) and the sequence of records returned by the mapping
  * function of  `mapWithState`.
  * @param prevStateRDD The previous MapWithStateRDD on whose StateMap data `this` RDD
   *                    will be created
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/rate/PIDRateEstimator.scala b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/rate/PIDRateEstimator.scala
index a73e6cc2cd..dc02062b9e 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/rate/PIDRateEstimator.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/rate/PIDRateEstimator.scala
@@ -26,7 +26,7 @@ import org.apache.spark.internal.Logging
  * case of Spark Streaming the error is the difference between the measured processing
  * rate (number of elements/processing delay) and the previous rate.
  *
- * @see https://en.wikipedia.org/wiki/PID_controller
+ * @see <a href="https://en.wikipedia.org/wiki/PID_controller">PID controller (Wikipedia)</a>
  *
  * @param batchIntervalMillis the batch duration, in milliseconds
  * @param proportional how much the correction should depend on the current
diff --git a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/rate/RateEstimator.scala b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/rate/RateEstimator.scala
index 7b2ef6881d..e4b9dffee0 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/scheduler/rate/RateEstimator.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/scheduler/rate/RateEstimator.scala
@@ -24,7 +24,7 @@ import org.apache.spark.streaming.Duration
  * A component that estimates the rate at which an `InputDStream` should ingest
  * records, based on updates at every batch completion.
  *
- * @see [[org.apache.spark.streaming.scheduler.RateController]]
+ * Please see `org.apache.spark.streaming.scheduler.RateController` for more details.
  */
 private[streaming] trait RateEstimator extends Serializable {