[diffusion] doc: update doc (#23052)

docs/diffusion/api/openai_api.md

@@ -154,7 +154,7 @@ curl -sS -L "http://localhost:30010/v1/images/<IMAGE_ID>/content" \
 
 The server implements a subset of the OpenAI Videos API under the `/v1/videos` namespace.
 
-**Create a video**
+**Create a video (text-to-video)**
 
 **Endpoint:** `POST /v1/videos`
 
@@ -184,6 +184,33 @@ curl -sS -X POST "http://localhost:30010/v1/videos" \
       }'
 ```
 
+**Create a video (image-to-video)**
+
+For I2V or TI2V models (e.g., Wan2.1 I2V, LTX-2.3 two-stage), pass an input image via multipart form upload or a reference URL.
+
+**Curl Example (multipart form upload):**
+
+```bash
+curl -sS -X POST "http://localhost:30010/v1/videos" \
+  -H "Authorization: Bearer sk-proj-1234567890" \
+  -F "prompt=A cat playing a piano" \
+  -F "input_reference=@input_image.png" \
+  -F "size=1280x720"
+```
+
+**Curl Example (reference URL):**
+
+```bash
+curl -sS -X POST "http://localhost:30010/v1/videos" \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer sk-proj-1234567890" \
+  -d '{
+        "prompt": "A cat playing a piano",
+        "reference_url": "https://example.com/input_image.png",
+        "size": "1280x720"
+      }'
+```
+
 **List videos**
 
 **Endpoint:** `GET /v1/videos`