[SPARK-18615][DOCS] Switch to multi-line doc to avoid a genjavadoc bug for backticks

## What changes were proposed in this pull request?

Currently, single line comment does not mark down backticks to `<code>..</code>` but prints as they are (`` `..` ``). For example, the line below:

```scala
/** Return an RDD with the pairs from `this` whose keys are not in `other`. */
```

So, we could work around this as below:

```scala
/**
 * Return an RDD with the pairs from `this` whose keys are not in `other`.
 */
```

- javadoc

  - **Before**
    ![2016-11-29 10 39 14](https://cloud.githubusercontent.com/assets/6477701/20693606/e64c8f90-b622-11e6-8dfc-4a029216e23d.png)

  - **After**
    ![2016-11-29 10 39 08](https://cloud.githubusercontent.com/assets/6477701/20693607/e7280d36-b622-11e6-8502-d2e21cd5556b.png)

- scaladoc (this one looks fine either way)

  - **Before**
    ![2016-11-29 10 38 22](https://cloud.githubusercontent.com/assets/6477701/20693640/12c18aa8-b623-11e6-901a-693e2f6f8066.png)

  - **After**
    ![2016-11-29 10 40 05](https://cloud.githubusercontent.com/assets/6477701/20693642/14eb043a-b623-11e6-82ac-7cd0000106d1.png)

I suspect this is related with SPARK-16153 and genjavadoc issue in ` typesafehub/genjavadoc#85`.

## How was this patch tested?

I found them via

```
grep -r "\/\*\*.*\`" . | grep .scala
````

and then checked if each is in the public API documentation with manually built docs (`jekyll build`) with Java 7.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #16050 from HyukjinKwon/javadoc-markdown.

1 files changed, 6 insertions, 2 deletions

diff --git a/streaming/src/main/scala/org/apache/spark/streaming/StateSpec.scala b/streaming/src/main/scala/org/apache/spark/streaming/StateSpec.scala
index c3b28bd516..dcd698c860 100644
--- a/streaming/src/main/scala/org/apache/spark/streaming/StateSpec.scala
+++ b/streaming/src/main/scala/org/apache/spark/streaming/StateSpec.scala
@@ -70,10 +70,14 @@ import org.apache.spark.util.ClosureCleaner
 @Experimental
 sealed abstract class StateSpec[KeyType, ValueType, StateType, MappedType] extends Serializable {
 
-  /** Set the RDD containing the initial states that will be used by `mapWithState` */
+  /**
+   * Set the RDD containing the initial states that will be used by `mapWithState`
+   */
   def initialState(rdd: RDD[(KeyType, StateType)]): this.type
 
-  /** Set the RDD containing the initial states that will be used by `mapWithState` */
+  /**
+   * Set the RDD containing the initial states that will be used by `mapWithState`
+   */
   def initialState(javaPairRDD: JavaPairRDD[KeyType, StateType]): this.type
 
   /**