[SPARK-26151][SQL][FOLLOWUP] Return partial results for bad CSV records

## What changes were proposed in this pull request?

Updated SQL migration guide according to changes in #23120

Closes #23235 from MaxGekk/failuresafe-partial-result-followup.

Lead-authored-by: Maxim Gekk <maxim.gekk@databricks.com>
Co-authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: MaxGekk

docs/sql-migration-guide-upgrade.md

@@ -35,6 +35,8 @@ displayTitle: Spark SQL Upgrading Guide
 
   - Since Spark 3.0, CSV datasource uses java.time API for parsing and generating CSV content. New formatting implementation supports date/timestamp patterns conformed to ISO 8601. To switch back to the implementation used in Spark 2.4 and earlier, set `spark.sql.legacy.timeParser.enabled` to `true`.
 
+  - In Spark version 2.4 and earlier, CSV datasource converts a malformed CSV string to a row with all `null`s in the PERMISSIVE mode. Since Spark 3.0, returned row can contain non-`null` fields if some of CSV column values were parsed and converted to desired types successfully.
+
 ## 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.