Add `:markdownhelp:` to `EngineArgs` docs so markdown docstrings render properly (#17124)

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

docs/source/serving/engine_args.md

@@ -16,6 +16,7 @@ Below, you can find an explanation of every engine argument:
     :func: _engine_args_parser
     :prog: vllm serve
     :nodefaultconst:
+    :markdownhelp:
 ```
 
 ## Async Engine Arguments
@@ -29,4 +30,5 @@ Additional arguments are available to the asynchronous engine which is used for
     :func: _async_engine_args_parser
     :prog: vllm serve
     :nodefaultconst:
+    :markdownhelp:
 ```

requirements/docs.txt

@@ -7,6 +7,7 @@ sphinx-togglebutton==0.3.2
 myst-parser==3.0.1
 msgspec
 cloudpickle
+commonmark # Required by sphinx-argparse when using :markdownhelp:
 
 # packages to install to build the documentation
 cachetools