Update preference text to 'Download referenced files (PDFs, ...)

[gepetton]

fixes #12560

This PR updates the text for the "Download linked online files" checkbox in JabRef's UI and synchronizes the user documentation to reflect the current behavior. The changes address feedback that the previous description was misleading

Link to Issue: #12560
Related Documentation PR: JabRef/user-documentation#552

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[github-actions]

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