tests/resource/aws_docdb_cluster: Address remaining #7122 feedback
tests/resource/aws_docdb_cluster: Address remaining #7122 feedback

Output from acceptance testing:

```
--- PASS: TestAccAWSDocDBCluster_missingUserNameCausesError (3.37s)
--- PASS: TestAccAWSDocDBCluster_generatedName (124.32s)
--- PASS: TestAccAWSDocDBCluster_encrypted (124.52s)
--- PASS: TestAccAWSDocDBCluster_basic (124.54s)
--- PASS: TestAccAWSDocDBCluster_namePrefix (127.33s)
--- PASS: TestAccAWSDocDBCluster_kmsKey (144.68s)
--- PASS: TestAccAWSDocDBCluster_updateCloudwatchLogsExports (160.52s)
--- PASS: TestAccAWSDocDBCluster_updateTags (161.27s)
--- PASS: TestAccAWSDocDBCluster_takeFinalSnapshot (187.32s)
--- PASS: TestAccAWSDocDBCluster_backupsUpdate (246.18s)
--- PASS: TestAccAWSDocDBCluster_Port (250.42s)
```

kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws