[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, 12 insertions, 4 deletions

diff --git a/mllib-local/src/main/scala/org/apache/spark/ml/linalg/Matrices.scala b/mllib-local/src/main/scala/org/apache/spark/ml/linalg/Matrices.scala
index 4d4b06b095..d9ffdeb797 100644
--- a/mllib-local/src/main/scala/org/apache/spark/ml/linalg/Matrices.scala
+++ b/mllib-local/src/main/scala/org/apache/spark/ml/linalg/Matrices.scala
@@ -85,11 +85,15 @@ sealed trait Matrix extends Serializable {
   @Since("2.0.0")
   def copy: Matrix
 
-  /** Transpose the Matrix. Returns a new `Matrix` instance sharing the same underlying data. */
+  /**
+   * Transpose the Matrix. Returns a new `Matrix` instance sharing the same underlying data.
+   */
   @Since("2.0.0")
   def transpose: Matrix
 
-  /** Convenience method for `Matrix`-`DenseMatrix` multiplication. */
+  /**
+   * Convenience method for `Matrix`-`DenseMatrix` multiplication.
+   */
   @Since("2.0.0")
   def multiply(y: DenseMatrix): DenseMatrix = {
     val C: DenseMatrix = DenseMatrix.zeros(numRows, y.numCols)
@@ -97,13 +101,17 @@ sealed trait Matrix extends Serializable {
     C
   }
 
-  /** Convenience method for `Matrix`-`DenseVector` multiplication. For binary compatibility. */
+  /**
+   * Convenience method for `Matrix`-`DenseVector` multiplication. For binary compatibility.
+   */
   @Since("2.0.0")
   def multiply(y: DenseVector): DenseVector = {
     multiply(y.asInstanceOf[Vector])
   }
 
-  /** Convenience method for `Matrix`-`Vector` multiplication. */
+  /**
+   * Convenience method for `Matrix`-`Vector` multiplication.
+   */
   @Since("2.0.0")
   def multiply(y: Vector): DenseVector = {
     val output = new DenseVector(new Array[Double](numRows))