[Yaml] improve error message for multiple documents

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

The YAML parser doesn't support multiple documents. This pull requests
improves the error message when the parser detects multiple YAML
documents.

see also #11840