Don't show default value for flags in `EngineArgs` (#4223)

Co-authored-by: Harry Mellor <hmellor@oxts.com>

docs/source/models/engine_args.rst

@@ -9,6 +9,7 @@ Below, you can find an explanation of every engine argument for vLLM:
     :module: vllm.engine.arg_utils
     :func: _engine_args_parser
     :prog: -m vllm.entrypoints.openai.api_server
+    :nodefaultconst:
 
 Async Engine Arguments
 ----------------------
@@ -18,4 +19,5 @@ Below are the additional arguments related to the asynchronous engine:
 .. argparse::
     :module: vllm.engine.arg_utils
     :func: _async_engine_args_parser
-    :prog: -m vllm.entrypoints.openai.api_server
\ No newline at end of file
+    :prog: -m vllm.entrypoints.openai.api_server
+    :nodefaultconst:
\ No newline at end of file