[Doc][skip ci] Add design doc for scheduler-managed diffusion paged KV cache (#6891)

Signed-off-by: Acerak01-fy <wfy2003324@163.com>

docs/user_guide/diffusion/execution_modes.md

@@ -108,7 +108,12 @@ set `DIFFUSION_ATTENTION_BACKEND=TORCH_SDPA` or configure
 `diffusion_attention_config.default.backend=TORCH_SDPA` before using
 `--max-num-seqs >1`. See the
 [HunyuanImage-3.0 recipe](https://github.com/vllm-project/vllm-omni/blob/main/recipes/Tencent/HunyuanImage-3.0-Instruct.md)
-for its validated configuration. Helios supports single-request step execution only: use
+for its validated configuration. For HunyuanImage3, this step-execution
+support applies to the `dense_legacy` path; `paged_scheduler` currently
+supports request-level execution only. See the
+[Scheduler-Managed Paged KV Cache guide](paged_kv_cache.md) for its required
+backend and configuration. Helios supports single-request step
+execution only: use
 `--step-execution --max-num-seqs 1` for Helios. MiniMax H3 supports step-wise
 continuous batching by packing co-batched requests into one sequence that keeps
 a separate attention document per request; that layout needs a backend which