[Misc] Improve comment for DPEngineCoreActor._set_cuda_visible_devices() (#21501)

Signed-off-by: Rui Qiao <ruisearch42@gmail.com>

vllm/v1/engine/core.py

@@ -1082,8 +1082,13 @@ class DPEngineCoreActor(DPEngineCoreProc):
         # RAY_EXPERIMENTAL_NOSET_CUDA_VISIBLE_DEVICES, but vLLM workers created
         # thereafter would have CUDA_VISIBLE_DEVICES set, which is sticky:
         # https://github.com/ray-project/ray/blob/e752fc319ddedd9779a0989b6d3613909bad75c9/python/ray/_private/worker.py#L456 # noqa: E501
-        # But vLLM worker assumes visibility into all local GPUs, therefore
-        # this results in incorrect indexing into the GPU ID list.
+        # This is problematic because when the vLLM worker (a Ray actor)
+        # executes a task, it indexes into the sticky CUDA_VISIBLE_DEVICES
+        # rather than directly using the GPU ID, potentially resulting in
+        # index out of bounds error. See:
+        # https://github.com/ray-project/ray/pull/40461/files#diff-31e8159767361e4bc259b6d9883d9c0d5e5db780fcea4a52ead4ee3ee4a59a78R1860 # noqa: E501
+        # and get_accelerator_ids_for_accelerator_resource() in worker.py
+        # of ray.
         self._set_cuda_visible_devices(vllm_config, local_dp_rank)
 
         super().__init__(vllm_config, local_client, "", executor_class,