[SPARK-25760][DOCS][FOLLOWUP] Add note about AddJar return value change in migration guide

## What changes were proposed in this pull request?

Add note about AddJar return value change in migration guide

## How was this patch tested?

n/a

Closes apache#22826 from srowen/SPARK-25760.2.

Authored-by: Sean Owen <sean.owen@databricks.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>

Author: srowen

docs/sql-migration-guide-upgrade.md

@@ -15,6 +15,8 @@ displayTitle: Spark SQL Upgrading Guide
 
   - Since Spark 3.0, the `from_json` functions supports two modes - `PERMISSIVE` and `FAILFAST`. The modes can be set via the `mode` option. The default mode became `PERMISSIVE`. In previous versions, behavior of `from_json` did not conform to either `PERMISSIVE` nor `FAILFAST`, especially in processing of malformed JSON records. For example, the JSON string `{"a" 1}` with the schema `a INT` is converted to `null` by previous versions but Spark 3.0 converts it to `Row(null)`.
 
+  - The `ADD JAR` command previously returned a result set with the single value 0. It now returns an empty result set.
+
 ## 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.