[DOC] Add missing docstring in LLMEngine.add_request() (vllm-project#12195)

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>

Author: terrytangyuan

vllm/engine/llm_engine.py

@@ -689,7 +689,9 @@ def add_request(
                 :class:`~vllm.PoolingParams` for pooling.
             arrival_time: The arrival time of the request. If None, we use
                 the current monotonic time.
+            lora_request: The LoRA request to add.
             trace_headers: OpenTelemetry trace headers.
+            prompt_adapter_request: The prompt adapter request to add.
             priority: The priority of the request.
                 Only applicable with priority scheduling.