[Docs] Reduce noise in docs and --help from the JSON tip (#22567)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/cli/README.md

@@ -16,6 +16,16 @@ Available Commands:
 vllm {chat,complete,serve,bench,collect-env,run-batch}
 ```
 
+When passing JSON CLI arguments, the following sets of arguments are equivalent:
+
+- `--json-arg '{"key1": "value1", "key2": {"key3": "value2"}}'`
+- `--json-arg.key1 value1 --json-arg.key2.key3 value2`
+
+Additionally, list elements can be passed individually using `+`:
+
+- `--json-arg '{"key4": ["value3", "value4", "value5"]}'`
+- `--json-arg.key4+ value3 --json-arg.key4+='value4,value5'`
+
 ## serve
 
 Start the vLLM OpenAI Compatible API server.