[Doc] Improve diffusion generation parameter docs for online serving (#2051)

Signed-off-by: samithuang <285365963@qq.com>
Signed-off-by: Samit <285365963@qq.com>
Signed-off-by: gcanlin <canlinguosdu@gmail.com>
Co-authored-by: Zeyu Huang | 黃澤宇 <11222265+fhfuih@users.noreply.github.com>
Co-authored-by: gcanlin <canlinguosdu@gmail.com>

docs/serving/diffusion_chat_api.md

@@ -0,0 +1,78 @@
+# Diffusion Chat Completions API
+
+vLLM-Omni supports generating and editing images via the `/v1/chat/completions`
+endpoint using diffusion models. This page explains how to pass generation
+parameters (such as `num_inference_steps`, `height`, `width`) to diffusion
+models through this endpoint.
+
+!!! tip
+    For dedicated endpoints that accept generation parameters as top-level
+    fields, see [Image Generation API](image_generation_api.md) and
+    [Image Edit API](image_edit_api.md).
+
+## Passing Generation Parameters
+
+The `/v1/chat/completions` endpoint follows the OpenAI Chat API schema, which
+does not natively include diffusion-specific fields like `num_inference_steps`
+or `height`. How you pass these extra fields depends on your client.
+
+### curl / Python `requests`
+
+Wrap generation parameters inside an `"extra_body"` key in the JSON body:
+
+```bash
+curl -s http://localhost:8091/v1/chat/completions \
+  -H "Content-Type: application/json" \
+  -d '{
+    "messages": [
+      {"role": "user", "content": "A beautiful landscape painting"}
+    ],
+    "extra_body": {
+      "num_inference_steps": 50,
+      "seed": 42
+    }
+  }'
+```
+
+### OpenAI Python SDK
+
+Use the `extra_body` **keyword argument**. The SDK automatically merges these
+fields into the top-level request body:
+
+```python
+response = client.chat.completions.create(
+    model="Qwen/Qwen-Image",
+    messages=[{"role": "user", "content": "A beautiful landscape painting"}],
+    extra_body={
+        "num_inference_steps": 50,
+        "seed": 42,
+    },
+)
+```
+
+!!! note "SDK `extra_body` vs. JSON `extra_body`"
+    These two `extra_body` usages look similar but work differently under the
+    hood. The SDK flattens the dict into the top-level request JSON, while the
+    curl/requests approach sends it as a nested `"extra_body"` key. Both are
+    handled correctly by the server.
+
+!!! note "About the `ignored fields` warning"
+    You may see a log message like:
+
+    ```
+    WARNING: The following fields were present in the request but ignored: {'height', 'width', ...}
+    ```
+
+    This is **harmless**. It is emitted by vLLM's request validation layer
+    because these fields are not part of the standard OpenAI
+    `ChatCompletionRequest` schema. The fields are still stored internally
+    and correctly forwarded to the diffusion pipeline.
+
+## Model-Specific Examples
+
+For complete examples with full request/response details, see the model-specific
+guides:
+
+- [Text-to-Image (Qwen-Image)](../user_guide/examples/online_serving/text_to_image.md)
+- [Image-to-Image (Qwen-Image-Edit, Qwen-Image-Layered)](../user_guide/examples/online_serving/image_to_image.md)
+- [GLM-Image](../user_guide/examples/online_serving/glm_image.md)