Improve-mm-and-pooler-and-decoding-configs (#16789)

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

tests/engine/test_arg_utils.py

@@ -24,6 +24,10 @@ from vllm.utils import FlexibleArgumentParser
     }),
 ])
 def test_limit_mm_per_prompt_parser(arg, expected):
+    """This functionality is deprecated and will be removed in the future.
+    This argument should be passed as JSON string instead.
+    
+    TODO: Remove with nullable_kvs."""
     parser = EngineArgs.add_cli_args(FlexibleArgumentParser())
     if arg is None:
         args = parser.parse_args([])