fix: add safety comment for type checking in join keys assignment

Author: kosiew

python/datafusion/dataframe.py

@@ -811,6 +811,7 @@ def _prepare_join(
             and isinstance(on[0], list)
             and isinstance(on[1], list)
         ):
+            # We know this is safe because we've checked the types            
             join_keys = on  # type: ignore[assignment]
             resolved_on = None
         else: