[Filebeat/Journald] Add note about the issues with using globs

[belimawr]

Proposed commit message

See title

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas~~
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

## Disruptive User Impact
## Author's Checklist

How to test this PR locally

Build the docs using docs-builder, then open http://localhost:3000/reference/filebeat/filebeat-input-journald#filebeat-input-journald-paths

Docs preview

• Docs preview

## Related issues
## Use cases
## Screenshots
## Logs

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[github-actions]

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• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[github-actions]

🔍 Preview links for changed docs

• docs/reference/filebeat/filebeat-input-journald.md