improve error message for multiple documents

The YAML parser doesn't support multiple documents. This pull requests
improves the error message when the parser detects multiple YAML
documents.
bug #12370 [Yaml] improve error message for multiple documents (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[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

Commits
-------

c77fdcb improve error message for multiple documents