[SPARK-4761] [DOC] [SQL] kryo default setting in SQL Thrift server

this is a follow up of apache#3621

/cc liancheng pwendell

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes apache#6639 from adrian-wang/kryodoc and squashes the following commits:

3c4b1cf [Daoyuan Wang] [DOC] kryo default setting in SQL Thrift server

docs/configuration.md

@@ -618,7 +618,7 @@ Apart from these, the following properties are also available, and may be useful
 </tr>
 <tr>
   <td><code>spark.kryo.referenceTracking</code></td>
-  <td>true</td>
+  <td>true (false when using Spark SQL Thrift Server)</td>
   <td>
     Whether to track references to the same object when serializing data with Kryo, which is
     necessary if your object graphs have loops and useful for efficiency if they contain multiple
@@ -679,7 +679,10 @@ Apart from these, the following properties are also available, and may be useful
 </tr>
 <tr>
   <td><code>spark.serializer</code></td>
-  <td>org.apache.spark.serializer.<br />JavaSerializer</td>
+  <td>
+    org.apache.spark.serializer.<br />JavaSerializer (org.apache.spark.serializer.<br />
+    KryoSerializer when using Spark SQL Thrift Server)
+  </td>
   <td>
     Class to use for serializing objects that will be sent over the network or need to be cached
     in serialized form. The default of Java serialization works with any Serializable Java object