[SPARK-20937][DOCS] Describe spark.sql.parquet.writeLegacyFormat prop…

…erty in Spark SQL, DataFrames and Datasets Guide

## What changes were proposed in this pull request?
Describe spark.sql.parquet.writeLegacyFormat property in Spark SQL, DataFrames and Datasets Guide.

## How was this patch tested?
N/A

Closes apache#22453 from seancxmao/SPARK-20937.

Authored-by: seancxmao <seancxmao@gmail.com>
Signed-off-by: hyukjinkwon <gurwls223@apache.org>

docs/sql-programming-guide.md

@@ -1004,6 +1004,17 @@ Configuration of Parquet can be done using the `setConf` method on `SparkSession
     </p>
   </td>
 </tr>
+<tr>
+  <td><code>spark.sql.parquet.writeLegacyFormat</code></td>
+  <td>false</td>
+  <td>
+    If true, data will be written in a way of Spark 1.4 and earlier. For example, decimal values
+    will be written in Apache Parquet's fixed-length byte array format, which other systems such as
+    Apache Hive and Apache Impala use. If false, the newer format in Parquet will be used. For
+    example, decimals will be written in int-based format. If Parquet output is intended for use
+    with systems that do not support this newer format, set to true.
+  </td>
+</tr>
 </table>
 
 ## ORC Files

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -451,8 +451,11 @@ object SQLConf {
       .createWithDefault(10)
 
   val PARQUET_WRITE_LEGACY_FORMAT = buildConf("spark.sql.parquet.writeLegacyFormat")
-    .doc("Whether to be compatible with the legacy Parquet format adopted by Spark 1.4 and prior " +
-      "versions, when converting Parquet schema to Spark SQL schema and vice versa.")
+    .doc("If true, data will be written in a way of Spark 1.4 and earlier. For example, decimal " +
+      "values will be written in Apache Parquet's fixed-length byte array format, which other " +
+      "systems such as Apache Hive and Apache Impala use. If false, the newer format in Parquet " +
+      "will be used. For example, decimals will be written in int-based format. If Parquet " +
+      "output is intended for use with systems that do not support this newer format, set to true.")
     .booleanConf
     .createWithDefault(false)