Update the SQL migration guide.

Author: MaxGekk

docs/sql-migration-guide.md

@@ -29,6 +29,8 @@ license: |
   - In Spark 3.1, SQL UI data adopts the `formatted` mode for the query plan explain results. To restore the behavior before Spark 3.0, you can set `spark.sql.ui.explainMode` to `extended`.
 
   - In Spark 3.1, `from_unixtime`, `unix_timestamp`,`to_unix_timestamp`, `to_timestamp` and `to_date` will fail if the specified datetime pattern is invalid. In Spark 3.0 or earlier, they result `NULL`.
+  
+  - In Spark 3.1, the Parquet, ORC, Avro and JSON datasources throw the exception `org.apache.spark.sql.AnalysisException: Found duplicate column(s) in the data schema` in read if they detect duplicate names in top-level columns as well in nested structures. The datasources take into account the SQL config `spark.sql.caseSensitive` while detecting column name duplicates.
 
 ## Upgrading from Spark SQL 3.0 to 3.0.1