Add service endpoint persistent connection problem tip

As provided by the "Diagnose and solve problems" in the Azure portal.

Author: cdhunt

articles/postgresql/howto-troubleshoot-common-connection-issues.md

@@ -44,6 +44,7 @@ If the application persistently fails to connect to Azure Database for PostgreSQ
 * Server firewall configuration: Make sure that the Azure Database for PostgreSQL server firewall is configured to allow connections from your client, including proxy servers and gateways.
 * Client firewall configuration: The firewall on your client must allow connections to your database server. IP addresses and ports of the server that you cannot to must be allowed as well as application names such as PostgreSQL in some firewalls.
 * User error: You might have mistyped connection parameters, such as the server name in the connection string or a missing *\@servername* suffix in the user name.
+* If you see the error _Server is not configured to allow ipv6 connections_, note that the Basic tier does not support VNet service endpoints. You have to remove the endpoint Microsoft.Sql from the subnet attempting to connect to the Basic server.
 
 ### Steps to resolve persistent connectivity issues