[SPARK-30312][DOCS][FOLLOWUP] Add a migration guide

This is a followup of #26956 to add a migration document for 2.4.5.

New legacy configuration will restore the previous behavior safely.

This PR updates the doc.

<img width="763" alt="screenshot" src="https://user-images.githubusercontent.com/9700541/72639939-9da5a400-391b-11ea-87b1-14bca15db5a6.png">

Build the document and see the change manually.

Closes #27269 from dongjoon-hyun/SPARK-30312.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
(cherry picked from commit fdbded3)
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: dongjoon-hyun

docs/sql-migration-guide-upgrade.md

@@ -14,6 +14,8 @@ displayTitle: Spark SQL Upgrading Guide
    `spark.sql.legacy.rdd.applyConf` to `false`: in this case, SQL configurations are ignored for operations
    performed on a RDD derived from a Dataset.
 
+ - Since Spark 2.4.5, `TRUNCATE TABLE` command tries to set back original permission and ACLs during re-creating the table/partition paths. To restore the behaviour of earlier versions, set `spark.sql.truncateTable.ignorePermissionAcl.enabled` to `true`.
+
 ## Upgrading from Spark SQL 2.4 to 2.4.1
 
   - The value of `spark.executor.heartbeatInterval`, when specified without units like "30" rather than "30s", was