Merge pull request hashicorp#5680 from iceycake/aws_db_instance-missing-tags-doc

Added the missing tags params to the aws_db_instance doc

Author: stack72

website/source/docs/providers/aws/r/db_instance.html.markdown

@@ -103,6 +103,7 @@ database, and to use this value as the source database. This correlates to the
 enhanced monitoring metrics to CloudWatch Logs. You can find more information on the [AWS Documentation](http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_Monitoring.html) 
 what IAM permissions are needed to allow Enhanced Monitoring for RDS Instances.
 * `monitoring_interval` - (Optional) The interval, in seconds, between points when Enhanced Monitoring metrics are collected for the DB instance. To disable collecting Enhanced Monitoring metrics, specify 0. The default is 0. Valid Values: 0, 1, 5, 10, 15, 30, 60.
+* `tags` - (Optional) A mapping of tags to assign to the resource.
 
 ~> **NOTE:** Removing the `replicate_source_db` attribute from an existing RDS
 Replicate database managed by Terraform will promote the database to a fully