[SPARK-28152][DOCS][FOLLOWUP] Add a migration guide for MsSQLServer J…

…DBC dialect

This PR adds a migration guide for MsSQLServer JDBC dialect for Apache Spark 2.4.4 and 2.4.5.

Apache Spark 2.4.4 updates the type mapping correctly according to MS SQL Server, but missed to mention that in the migration guide. In addition, 2.4.4 adds a configuration for the legacy behavior.

Yes. This is a documentation change.

![screenshot](https://user-images.githubusercontent.com/9700541/72649944-d6517780-3933-11ea-92be-9d4bf38e2eda.png)

Manually generate and see the doc.

Closes #27270 from dongjoon-hyun/SPARK-28152-DOC.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
(cherry picked from commit 505693c)
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

docs/sql-migration-guide-upgrade.md

@@ -16,6 +16,12 @@ displayTitle: Spark SQL Upgrading Guide
 
  - 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`.
 
+  - Since Spark 2.4.5, `spark.sql.legacy.mssqlserver.numericMapping.enabled` configuration is added in order to support the legacy MsSQLServer dialect mapping behavior using IntegerType and DoubleType for SMALLINT and REAL JDBC types, respectively. To restore the behaviour of 2.4.3 and earlier versions, set `spark.sql.legacy.mssqlserver.numericMapping.enabled` to `true`.
+
+## Upgrading from Spark SQL 2.4.3 to 2.4.4
+
+    - Since Spark 2.4.4, according to [MsSqlServer Guide](https://docs.microsoft.com/en-us/sql/connect/jdbc/using-basic-data-types?view=sql-server-2017), MsSQLServer JDBC Dialect uses ShortType and FloatType for SMALLINT and REAL, respectively. Previously, IntegerType and DoubleType is used.
+
 ## 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