chore: Update config.sample.php to document new ffprobe path option

[invario]

PR #51712 added new preview_ffprobe_path parameter. This PR updates config.sample.php to document this new optional parameter and the fallback behavior if the path is not set manually.

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[st3iny]

Cypress and performance tests won't run as this is a fork. Either someone needs to import the PR or we just force merge (changes are unrelated to both).

[github-actions]

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