docs: clarify benchmark model settings option

[shijian0-eng]

Summary

• Clarify that the benchmark script uses --read-model-settings=<filename.yml>.
• Distinguish it from the main aider CLI option, --model-settings-file .
• Cross-reference the model settings documentation.

Fixes #5131

Scope

This is a docs-only change. benchmark/benchmark.py is intentionally unchanged.

Tests

• D:\pytorchAnaconda\envs\pytorch2\python.exe benchmark/benchmark.py --help with PYTHONPATH=.: passed
• D:\pytorchAnaconda\envs\pytorch2\python.exe -m pip check: passed
• git diff --check: passed

[shijian0-eng]

Hi maintainers, a quick follow-up: the CI workflows expired while awaiting approval, so no jobs were run.

This PR only clarifies the model settings options in the documentation; benchmark/benchmark.py is unchanged. When you have a chance, could you please help re-trigger and approve the workflows?

Thank you for your time!