[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.

2 files changed, 3 insertions, 3 deletions

diff --git a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveQueryFileTest.scala b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveQueryFileTest.scala
index e772324a57..bb4ce6d3aa 100644
--- a/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveQueryFileTest.scala
+++ b/sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveQueryFileTest.scala
@@ -24,7 +24,7 @@ import org.apache.spark.sql.catalyst.util._
 /**
  * A framework for running the query tests that are listed as a set of text files.
  *
- * TestSuites that derive from this class must provide a map of testCaseName -> testCaseFiles
+ * TestSuites that derive from this class must provide a map of testCaseName to testCaseFiles
  * that should be included. Additionally, there is support for whitelisting and blacklisting
  * tests as development progresses.
  */
diff --git a/sql/hive/src/test/scala/org/apache/spark/sql/hive/orc/OrcTest.scala b/sql/hive/src/test/scala/org/apache/spark/sql/hive/orc/OrcTest.scala
index 7226ed521e..a2f08c5ba7 100644
--- a/sql/hive/src/test/scala/org/apache/spark/sql/hive/orc/OrcTest.scala
+++ b/sql/hive/src/test/scala/org/apache/spark/sql/hive/orc/OrcTest.scala
@@ -43,7 +43,7 @@ private[sql] trait OrcTest extends SQLTestUtils with TestHiveSingleton {
   }
 
   /**
-   * Writes `data` to a Orc file and reads it back as a [[DataFrame]],
+   * Writes `data` to a Orc file and reads it back as a `DataFrame`,
    * which is then passed to `f`. The Orc file will be deleted after `f` returns.
    */
   protected def withOrcDataFrame[T <: Product: ClassTag: TypeTag]
@@ -53,7 +53,7 @@ private[sql] trait OrcTest extends SQLTestUtils with TestHiveSingleton {
   }
 
   /**
-   * Writes `data` to a Orc file, reads it back as a [[DataFrame]] and registers it as a
+   * Writes `data` to a Orc file, reads it back as a `DataFrame` and registers it as a
    * temporary table named `tableName`, then call `f`. The temporary table together with the
    * Orc file will be dropped/deleted after `f` returns.
    */