[doc][misc] add note for Kubernetes users (#5916)

Author: youkaichao

docs/source/serving/env_vars.rst

@@ -6,6 +6,8 @@ vLLM uses the following environment variables to configure the system:
 .. warning::
     Please note that ``VLLM_PORT`` and ``VLLM_HOST_IP`` set the port and ip for vLLM's **internal usage**. It is not the port and ip for the API server. If you use ``--host $VLLM_HOST_IP`` and ``--port $VLLM_PORT`` to start the API server, it will not work.
 
+    All environment variables used by vLLM are prefixed with ``VLLM_``. **Special care should be taken for Kubernetes users**: please do not name the service as ``vllm``, otherwise environment variables set by Kubernetes might conflict with vLLM's environment variables, because `Kubernetes sets environment variables for each service with the capitalized service name as the prefix <https://kubernetes.io/docs/concepts/services-networking/service/#environment-variables>`_.
+
 .. literalinclude:: ../../../vllm/envs.py
     :language: python
     :start-after: begin-env-vars-definition