[Doc] Add environment variables configuration reference (#6217)

Signed-off-by: Hongsheng Liu <liuhongsheng4@huawei.com>

docs/cli/README.md

@@ -2,6 +2,11 @@
 
 The CLI for vLLM-Omni inherits from vllm with some additional arguments.
 
+Environment variables are process configuration rather than CLI-only options.
+See the [Environment Variables](../configuration/environment_variables.md)
+reference for ownership, precedence, evaluation time, and per-stage `env`
+scoping.
+
 ## serve
 
 Starts the vLLM-Omni OpenAI Compatible API server.