[V1] Logits processor docs (#22919)

Signed-off-by: Andrew Feldman <afeldman@redhat.com>
Signed-off-by: afeldman-nm <156691304+afeldman-nm@users.noreply.github.com>
Co-authored-by: Joseph Marinier <Joseph.Marinier@gmail.com>

docs/features/custom_arguments.md

@@ -0,0 +1,46 @@
+# Custom Arguments
+
+You can use vLLM *custom arguments* to pass in arguments which are not part of the vLLM `SamplingParams` and REST API specifications. Adding or removing a vLLM custom argument does not require recompiling vLLM, since the custom arguments are passed in as a dictionary.
+
+Custom arguments can be useful if, for example, you want to use a [custom logits processor](./custom_logitsprocs.md) without modifying the vLLM source code.
+
+## Offline Custom Arguments
+
+Custom arguments passed to `SamplingParams.extra_args` as a `dict` will be visible to any code which has access to `SamplingParams`:
+
+``` python
+SamplingParams(extra_args={"your_custom_arg_name": 67})
+```
+
+This allows arguments which are not already part of `SamplingParams` to be passed into `LLM` as part of a request.
+
+## Online Custom Arguments
+
+The vLLM REST API allows custom arguments to be passed to the vLLM server via `vllm_xargs`. The example below integrates custom arguments into a vLLM REST API request:
+
+``` bash
+curl http://localhost:8000/v1/completions \
+    -H "Content-Type: application/json" \
+    -d '{
+        "model": "Qwen/Qwen2.5-1.5B-Instruct",
+        ...
+        "vllm_xargs": {"your_custom_arg": 67}
+    }'
+```
+
+Furthermore, OpenAI SDK users can access `vllm_xargs` via the `extra_body` argument:
+
+``` python
+batch = await client.completions.create(
+    model="Qwen/Qwen2.5-1.5B-Instruct",
+    ...,
+    extra_body={
+        "vllm_xargs": {
+            "your_custom_arg": 67
+        }
+    }
+)
+```
+
+!!! note
+    `vllm_xargs` is assigned to `SamplingParams.extra_args` under the hood, so code which uses `SamplingParams.extra_args` is compatible with both offline and online scenarios.