Allow users to pass arbitrary JSON keys from CLI (#18208)

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

vllm/engine/arg_utils.py

@@ -183,7 +183,11 @@ def get_kwargs(cls: ConfigType) -> dict[str, Any]:
         kwargs[name] = {"default": default, "help": help}
 
         # Set other kwargs based on the type hints
-        json_tip = "\n\nShould be a valid JSON string."
+        json_tip = """\n\nShould either be a valid JSON string or JSON keys
+        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"""
         if dataclass_cls is not None:
             dataclass_init = lambda x, f=dataclass_cls: f(**json.loads(x))
             # Special case for configs with a from_cli method