Update num_candidates description in KNN query documentation

[daixque]

Clarified the behavior of num_candidates when using rescore_vector with quantized vectors.

Source:
https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java#L2773

[elasticsearchmachine]

@daixque please enable the option "Allow edits and access to secrets by maintainers" on your PR. For more information, see the documentation.

[github-actions]

🔍 Preview links for changed docs

• docs/reference/query-languages/query-dsl/query-dsl-knn-query.md

[github-actions]

ℹ️ Important: Docs version tagging

👋 Thanks for updating the docs! Just a friendly reminder that our docs are now cumulative. This means all 9.x versions are documented on the same page and published off of the main branch, instead of creating separate pages for each minor version.

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