docs(vllm-omni): note transformers>=5.0 requirement for GLM-Image (#8287)

Signed-off-by: ayushag <ayushag@nvidia.com>

docs/backends/vllm/vllm-omni.md

@@ -372,6 +372,8 @@ sequenceDiagram
 
 GLM-Image is a 2-stage text-to-image model with an AR stage (generates prior token IDs) and a DiT stage (diffusion denoising + VAE decode). The built-in vLLM-Omni stage config already assigns each stage to a separate GPU.
 
+> **Known issue:** GLM-Image requires `transformers>=5.0` to recognize the `glm_image` architecture. Older versions fail at model config creation with `The checkpoint you are trying to load has model type 'glm_image' but Transformers does not recognize this architecture`.
+
 ```bash
 bash examples/backends/vllm/launch/disagg_omni_glm_image.sh
 ```