docs: document min confidence edge cases

[vku2018]

Summary:

• document min_confidence in the README span-output section with a short example
• note that transformer span confidences start at 0.5, so lower thresholds are no-ops
• clarify that RAGFactCheckerDetector.predict(..., output_format="detailed") is not filtered by min_confidence

Validation:

• python -m compileall lettucedetect\detectors\rag_fact_checker.py
• python -m ruff check README.md lettucedetect\detectors\rag_fact_checker.py
• git diff --check

Closes #64

[adaamko]

Thanks — this is exactly what #64 asked for, and I verified all three claims against the code (the ≥ 0.5 transformer confidence floor, the unfiltered detailed output for RAGFactChecker, and the README example). Content-wise I'd merge as-is. Two process items: please mark the PR ready for review so CI runs, and add the contribution-rights line to the description and tick it:

- [x] I certify that I have the right to submit this code and that it may be distributed under the repository's MIT license

Once checks are green this goes in.