Add explanation

Author: dandavison

pyproject.toml

@@ -76,7 +76,7 @@ test = "uv run pytest"
 [tool.ruff]
 target-version = "py39"
 exclude = ["*_pb2.py", "*_pb2_grpc.py"]
-lint.ignore = ["E741"]
+lint.ignore = ["E741"]  # we occasionally use e.g. O as a type var and l as a loop variable
 
 
 [tool.pytest.ini_options]