[Entrypoints] Support Qwen-Image-Edit 2509 online serving & update doc (#368)

Signed-off-by: David Chen <530634352@qq.com>

docs/user_guide/acceleration/cache_dit_acceleration.md

@@ -50,6 +50,22 @@ omni = Omni(
 )
 ```
 
+## Online Serving (OpenAI-Compatible)
+
+Enable Cache-DiT for online serving by passing `--cache-backend cache_dit` when starting the server:
+
+```bash
+# Use Cache-DiT default (recommended) parameters
+vllm serve Qwen/Qwen-Image --omni --port 8091 --cache-backend cache_dit
+```
+
+To customize Cache-DiT settings for online serving, pass a JSON string via `--cache-config`:
+
+```bash
+vllm serve Qwen/Qwen-Image --omni --port 8091 \
+  --cache-backend cache_dit \
+  --cache-config '{"Fn_compute_blocks": 1, "Bn_compute_blocks": 0, "max_warmup_steps": 4, "residual_diff_threshold": 0.12}'
+```
 
 ## Acceleration Methods