Update the migration guide.

apache · Nov 14, 2018 · e19b1e4 · e19b1e4
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diff --git a/docs/sql-migration-guide-upgrade.md b/docs/sql-migration-guide-upgrade.md
@@ -17,6 +17,8 @@ displayTitle: Spark SQL Upgrading Guide
 
   - The `ADD JAR` command previously returned a result set with the single value 0. It now returns an empty result set.
 
+  - In Spark version 2.4 and earlier, the `SET` command works without any warnings even if the specified key is for `SparkConf` entries and it has no effect because the command does not update `SparkConf`, but the behavior might confuse users. Since 3.0, the command fails if the key is for `SparkConf` as same as for static sql config keys. You can disable such a check by setting `spark.sql.execution.setCommandRejectsSparkConfs` to `false`.
+
 ## Upgrading From Spark SQL 2.3 to 2.4
 
   - In Spark version 2.3 and earlier, the second parameter to array_contains function is implicitly promoted to the element type of first array type parameter. This type promotion can be lossy and may cause `array_contains` function to return wrong result. This problem has been addressed in 2.4 by employing a safer type promotion mechanism. This can cause some change in behavior and are illustrated in the table below.