Issue title: Clarify dashboard provisioning name
 Issue body: It seems like the dashboard provisioner name must be unique, but no word about that in the example configuration or in the logfiles when you use the same name multiple times.

```
apiVersion: 1

providers:
  # <string> provider name
- name: 'default'
```

The example `default` does not look like you need to change it.

What happens if you use the same name multiple times is a bit random. Sometimes the provisioned dashboards appear, but if you open them you get a "dashboard not found" error. Sometimes after multiple reloads they finally load.

**Suggestions**
1. Either remove the "name" parameter completly. Right now, it does not look like it adds any value.
2. at least log an error if the same name is used twice.
 Comments: 
 Comment 0: Thank you for reporting this, we'll try to update the documentation.
{'source': 'GithubIssues', 'repo_name_with_owner': 'grafana/grafana', 'issue_id': 454594684, 'repo_stars': 51867.0}