Add documentation for aws_db_instance/final_snapshot_identifer

Ran into an error today that revealed a `FinalDBSnapshotIdentifier`:

```
$ terraform apply -var-file terraform.tfvars
aws_db_security_group.azavea-postgresql: Refreshing state... (ID: azavea-postgresql)
aws_db_instance.azavea-postgresql-01: Refreshing state... (ID: azavea-postgresql-01)
aws_db_instance.azavea-postgresql-01: Destroying...
aws_db_instance.azavea-postgresql-01: Error: InvalidParameterCombination: FinalDBSnapshotIdentifier is required unless SkipFinalSnapshot is specified.
Error applying plan:

1 error(s) occurred:

* InvalidParameterCombination: FinalDBSnapshotIdentifier is required unless SkipFinalSnapshot is specified.

Terraform does not automatically rollback in the face of errors.
Instead, your Terraform state file has been partially updated with
any resources that successfully completed. Please address the error
above and apply again to incrementally change your infrastructure.
```

Tracked it down to https://github.com/hashicorp/terraform/issues/112. This pull request adds a reference to the documentation.