[Doc] Update plugin doc (#28532)

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

vllm/plugins/__init__.py

@@ -17,6 +17,9 @@ IO_PROCESSOR_PLUGINS_GROUP = "vllm.io_processor_plugins"
 # Platform plugins group will be loaded in all processes when
 # `vllm.platforms.current_platform` is called and the value not initialized,
 PLATFORM_PLUGINS_GROUP = "vllm.platform_plugins"
+# Stat logger plugins group will be loaded in process0 only when serve vLLM with
+# async mode.
+STAT_LOGGER_PLUGINS_GROUP = "vllm.stat_logger_plugins"
 
 # make sure one process only loads plugins once
 plugins_loaded = False