Updating the migration guide

Author: MaxGekk

docs/sql-migration-guide-upgrade.md

@@ -9,6 +9,8 @@ displayTitle: Spark SQL Upgrading Guide
 
 ## Upgrading From Spark SQL 2.4 to 3.0
 
+  - Since Spark 3.0, to parse decimals in locale specific format from CSV, set the `locale` option to proper value.
+   
   - In PySpark, when creating a `SparkSession` with `SparkSession.builder.getOrCreate()`, if there is an existing `SparkContext`, the builder was trying to update the `SparkConf` of the existing `SparkContext` with configurations specified to the builder, but the `SparkContext` is shared by all `SparkSession`s, so we should not update them. Since 3.0, the builder comes to not update the configurations. This is the same behavior as Java/Scala API in 2.3 and above. If you want to update them, you need to update them prior to creating a `SparkSession`.
 
   - In Spark version 2.4 and earlier, the parser of JSON data source treats empty strings as null for some data types such as `IntegerType`. For `FloatType` and `DoubleType`, it fails on empty strings and throws exceptions. Since Spark 3.0, we disallow empty strings and will throw exceptions for data types except for `StringType` and `BinaryType`.