match the doc in SQLConf with programming guide

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

@@ -474,8 +474,16 @@ 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("This configuration indicates whether we should use legacy Parquet format adopted by " +
+      "Spark 1.4 and prior versions or the standard format defined in parquet-format " +
+      "specification to write Parquet files. This is not only related to compatibility with old " +
+      "Spark ones, but also other systems like Hive, Impala, Presto, etc. This is especially " +
+      "important for decimals. If this configuration is not enabled, decimals will be written in " +
+      "int-based format in Spark 1.5 and above, other systems that only support legacy decimal " +
+      "format (fixed length byte array) will not be able to read what Spark has written. Note " +
+      "other systems may have added support for the standard format in more recent versions, " +
+      "which will make this configuration unnecessary. Please consult documentation of related " +
+      "systems for details.")
     .booleanConf
     .createWithDefault(false)