[SPARK-30200][DOCS][FOLLOW-UP] Add documentation for explain(mode: String)

### What changes were proposed in this pull request?

This PR adds the documentation of the new `mode` added to `Dataset.explain`.

### Why are the changes needed?

To let users know the new modes.

### Does this PR introduce any user-facing change?

No (doc-only change).

### How was this patch tested?

Manually built the doc:
![Screen Shot 2019-12-16 at 3 34 28 PM](https://user-images.githubusercontent.com/6477701/70884617-d64f1680-2019-11ea-9336-247ade7f8768.png)

Closes #26903 from HyukjinKwon/SPARK-30200-doc.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala

@@ -524,6 +524,16 @@ class Dataset[T] private[sql](
   /**
    * Prints the plans (logical and physical) with a format specified by a given explain mode.
    *
+   * @param mode specifies the expected output format of plans.
+   *             <ul>
+   *               <li>`simple` Print only a physical plan.</li>
+   *               <li>`extended`: Print both logical and physical plans.</li>
+   *               <li>`codegen`: Print a physical plan and generated codes if they are
+   *                 available.</li>
+   *               <li>`cost`: Print a logical plan and statistics if they are available.</li>
+   *               <li>`formatted`: Split explain output into two sections: a physical plan outline
+   *                 and node details.</li>
+   *             </ul>
    * @group basic
    * @since 3.0.0
    */
@@ -541,6 +551,8 @@ class Dataset[T] private[sql](
   /**
    * Prints the plans (logical and physical) to the console for debugging purposes.
    *
+   * @param extended default `false`. If `false`, prints only the physical plan.
+   *
    * @group basic
    * @since 1.6.0
    */