[doc] update usage of env var to avoid conflict (vllm-project#5873)

Author: youkaichao

docs/source/serving/env_vars.rst

@@ -3,6 +3,9 @@ Environment Variables
 
 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.
+
 .. literalinclude:: ../../../vllm/envs.py
     :language: python
     :start-after: begin-env-vars-definition