Adding firewall prerequisite information (#13385)

While trying to follow the example in this page I was getting GatewayTimeout errors. After updating the firewall to allow Azure services the examples work fine. This is important information to the success of the reader executing the example code. As this is essential information required for user success it has been highlighted as Important.

Author: johnmart82

src/Sql/Sql/help/New-AzSqlDatabaseExport.md

@@ -27,6 +27,9 @@ The **New-AzSqlDatabaseExport** cmdlet exports an Azure SQL Database as a .bacpa
 The get export database status request may be sent to retrieve status information for this request.
 This cmdlet is also supported by the SQL Server Stretch Database service on Azure.
 
+> [!IMPORTANT]
+> In order to make use of this cmdlet the firewall on the Azure SQL Server will need to be configured to "Allow Azure services and resources to access this server". If this is not configured then GatewayTimeout errors will be experienced.
+
 ## EXAMPLES
 
 ### Example 1: Create an export request for a database