[CLI] Improve CLI arg parsing for -O/--compilation-config (#20156)

Signed-off-by: luka <luka@neuralmagic.com>

vllm/engine/arg_utils.py

@@ -202,7 +202,10 @@ def _compute_kwargs(cls: ConfigType) -> dict[str, Any]:
         passed individually. For example, the following sets of arguments are
         equivalent:\n\n
         - `--json-arg '{"key1": "value1", "key2": {"key3": "value2"}}'`\n
-        - `--json-arg.key1 value1 --json-arg.key2.key3 value2`\n\n"""
+        - `--json-arg.key1 value1 --json-arg.key2.key3 value2`\n
+        Additionally, list elements can be passed individually using '+':
+        - `--json-arg '{"key4": ["value3", "value4", "value5"]}'`\n
+        - `--json-arg.key4+ value3 --json-arg.key4+='value4,value5'`\n\n"""
         if dataclass_cls is not None:
 
             def parse_dataclass(val: str, cls=dataclass_cls) -> Any: