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