[SPARK-15126][SQL] RuntimeConfig.set should return Unit

## What changes were proposed in this pull request?
Currently we return RuntimeConfig itself to facilitate chaining. However, it makes the output in interactive environments (e.g. notebooks, scala repl) weird because it'd show the response of calling set as a RuntimeConfig itself.

## How was this patch tested?
Updated unit tests.

Author: Reynold Xin <rxin@databricks.com>

Closes #12902 from rxin/SPARK-15126.

python/pyspark/sql/conf.py

@@ -23,7 +23,6 @@ class RuntimeConfig(object):
     """User-facing configuration API, accessible through `SparkSession.conf`.
 
     Options set here are automatically propagated to the Hadoop configuration during I/O.
-    This a thin wrapper around its Scala implementation org.apache.spark.sql.RuntimeConfig.
     """
 
     def __init__(self, jconf):

python/pyspark/sql/session.py

@@ -71,9 +71,6 @@ class SparkSession(object):
             .config("spark.some.config.option", "some-value") \
             .getOrCreate()
 
-    :param sparkContext: The :class:`SparkContext` backing this SparkSession.
-    :param jsparkSession: An optional JVM Scala SparkSession. If set, we do not instantiate a new
-        SparkSession in the JVM, instead we make all calls to this object.
     """
 
     class Builder(object):

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

@@ -35,9 +35,8 @@ class RuntimeConfig private[sql](sqlConf: SQLConf = new SQLConf) {
    *
    * @since 2.0.0
    */
-  def set(key: String, value: String): RuntimeConfig = {
+  def set(key: String, value: String): Unit = {
     sqlConf.setConfString(key, value)
-    this
   }
 
   /**
@@ -45,7 +44,7 @@ class RuntimeConfig private[sql](sqlConf: SQLConf = new SQLConf) {
    *
    * @since 2.0.0
    */
-  def set(key: String, value: Boolean): RuntimeConfig = {
+  def set(key: String, value: Boolean): Unit = {
     set(key, value.toString)
   }
 
@@ -54,7 +53,7 @@ class RuntimeConfig private[sql](sqlConf: SQLConf = new SQLConf) {
    *
    * @since 2.0.0
    */
-  def set(key: String, value: Long): RuntimeConfig = {
+  def set(key: String, value: Long): Unit = {
     set(key, value.toString)
   }
 

sql/core/src/test/scala/org/apache/spark/sql/internal/RuntimeConfigSuite.scala → sql/core/src/test/scala/org/apache/spark/sql/RuntimeConfigSuite.scala

@@ -15,10 +15,9 @@
  * limitations under the License.
  */
 
-package org.apache.spark.sql.internal
+package org.apache.spark.sql
 
 import org.apache.spark.SparkFunSuite
-import org.apache.spark.sql.RuntimeConfig
 
 class RuntimeConfigSuite extends SparkFunSuite {
 
@@ -26,10 +25,9 @@ class RuntimeConfigSuite extends SparkFunSuite {
 
   test("set and get") {
     val conf = newConf()
-    conf
-      .set("k1", "v1")
-      .set("k2", 2)
-      .set("k3", value = false)
+    conf.set("k1", "v1")
+    conf.set("k2", 2)
+    conf.set("k3", value = false)
 
     assert(conf.get("k1") == "v1")
     assert(conf.get("k2") == "2")
@@ -41,13 +39,15 @@ class RuntimeConfigSuite extends SparkFunSuite {
   }
 
   test("getOption") {
-    val conf = newConf().set("k1", "v1")
+    val conf = newConf()
+    conf.set("k1", "v1")
     assert(conf.getOption("k1") == Some("v1"))
     assert(conf.getOption("notset") == None)
   }
 
   test("unset") {
-    val conf = newConf().set("k1", "v1")
+    val conf = newConf()
+    conf.set("k1", "v1")
     assert(conf.get("k1") == "v1")
     conf.unset("k1")
     intercept[NoSuchElementException] {