Fix `autodoc` directives (#4272)

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

docs/source/dev/engine/async_llm_engine.rst

-
 AsyncLLMEngine
 =================================
 
-.. autoclass:: vllm.engine.async_llm_engine.AsyncLLMEngine
-    :members: generate, abort
+.. autoclass:: vllm.AsyncLLMEngine
+    :members:
     :show-inheritance:

docs/source/dev/engine/llm_engine.rst

 LLMEngine
 =================================
 
-.. autoclass:: vllm.engine.llm_engine.LLMEngine
-    :members: add_request, abort_request, step
-    :show-inheritance:
\ No newline at end of file
+.. autoclass:: vllm.LLMEngine
+    :members:
+    :show-inheritance:

docs/source/dev/sampling_params.rst

 Sampling Params
 ===============
 
-.. automodule:: vllm.sampling_params.SamplingParams
\ No newline at end of file
+.. autoclass:: vllm.SamplingParams
+    :members: