[doc][misc] further lower visibility of simple api server (#6041)

Co-authored-by: Simon Mo <simon.mo@hey.com>

examples/api_client.py

-"""Example Python client for vllm.entrypoints.api_server"""
+"""Example Python client for vllm.entrypoints.api_server
+NOTE: The API server is used only for demonstration and simple performance
+benchmarks. It is not intended for production use.
+For production use, we recommend vllm.entrypoints.openai.api_server
+and the OpenAI client API
+"""
 
 import argparse
 import json