Added guidance on retrieval similarity thresholds

[NatalieSasha]

I have used AI tools to help refine the wording of a single recommendation regarding the use of minimum similarity score thresholds during document retrieval in the RAG Security Cheat Sheet. I independently reviewed the final wording, verified the technical accuracy of the recommendation, and confirmed that any supporting references appropriately support the claim.

The LLM used is claude-opus-4-8.

Prompt used:

"I am contributing to the OWASP RAG Security Cheat Sheet. I want to add a recommendation that advises configuring a minimum similarity score threshold during retrieval to reduce the inclusion of low-confidence or adversarial documents. Help me refine the wording so it is concise, technically accurate, and consistent with the style of the existing cheat sheet."

I independently reviewed and verified the final recommendation and any cited references before submitting this pull request.