Added two CW log export values

The new values are supported by PostgreSQL after version PostgreSQL 9.6.6.  PostgreSQL doesn't support the existing values in the log export list.

website/docs/r/db_instance.html.markdown

@@ -94,7 +94,7 @@ for additional read replica contraints.
 * `deletion_protection` - (Optional) If the DB instance should have deletion protection enabled. The database can't be deleted when this value is set to `true`. The default is `false`.
 * `domain` - (Optional) The ID of the Directory Service Active Directory domain to create the instance in.
 * `domain_iam_role_name` - (Optional, but required if domain is provided) The name of the IAM role to be used when making API calls to the Directory Service.
-* `enabled_cloudwatch_logs_exports` - (Optional) List of log types to enable for exporting to CloudWatch logs. If omitted, no logs will be exported. Valid values (depending on `engine`): `alert`, `audit`, `error`, `general`, `listener`, `slowquery`, `trace`.
+* `enabled_cloudwatch_logs_exports` - (Optional) List of log types to enable for exporting to CloudWatch logs. If omitted, no logs will be exported. Valid values (depending on `engine`): `alert`, `audit`, `error`, `general`, `listener`, `slowquery`, `trace`, `postgresql` (PostgreSQL), `upgrade` (PostgreSQL).
 * `engine` - (Required unless a `snapshot_identifier` or `replicate_source_db`
 is provided) The database engine to use.  For supported values, see the Engine parameter in [API action CreateDBInstance](https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_CreateDBInstance.html).
 Note that for Amazon Aurora instances the engine must match the [DB cluster](/docs/providers/aws/r/rds_cluster.html)'s engine'.