[DOCS] Clarify negative scores returned by Elastic Rerank

[charlotte-hoblik]

• Added a note clarifying that relevance scores produced during reranking can include negative values.
• Removed "min_score": 0.5 from the example to avoid confusion, as users might incorrectly assume scores are always positive.
  Based on https://github.com/elastic/search-docs-team/issues/234

[github-actions]

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[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[peter-strsr]

I think this may have changed, since this was initially flagged.
@jimczi can scores for Elastic Rerank / the text similarity retriever still be negative?

[jimczi]

can scores for Elastic Rerank / the text similarity retriever still be negative?

This was considered a bug in previous versions and fixed in: #120930.
So negative scores are not possible anymore starting from 8.16.4.