[Doc] Minor v1_user_guide update (#15064)

Co-authored-by: Roger Wang <[email protected]>

Author: JenZhao

docs/source/getting_started/v1_user_guide.md

@@ -2,6 +2,8 @@
 
 V1 is now enabled by default for all supported use cases, and we will gradually enable it for every use case we plan to support. Please share any feedback on [GitHub](https://github.com/vllm-project/vllm) or in the [vLLM Slack](https://inviter.co/vllm-slack).
 
+To disable V1, please set the environment variable as: `VLLM_USE_V1=0`, and send us a GitHub issue sharing the reason!
+
 ## Why vLLM V1?
 
 vLLM V0 successfully supported a wide range of models and hardware, but as new features were developed independently, the system grew increasingly complex. This complexity made it harder to integrate new capabilities and introduced technical debt, revealing the need for a more streamlined and unified design.