[doc] add doc for the plugin system (#10372)

Signed-off-by: youkaichao <youkaichao@gmail.com>

vllm/plugins/__init__.py

@@ -27,16 +27,24 @@ def load_general_plugins():
     allowed_plugins = envs.VLLM_PLUGINS
 
     discovered_plugins = entry_points(group='vllm.general_plugins')
+    logger.info("Available plugins:")
+    for plugin in discovered_plugins:
+        logger.info("name=%s, value=%s, group=%s", plugin.name, plugin.value,
+                    plugin.group)
+    if allowed_plugins is None:
+        logger.info("all available plugins will be loaded.")
+        logger.info("set environment variable VLLM_PLUGINS to control"
+                    " which plugins to load.")
+    else:
+        logger.info("plugins to load: %s", allowed_plugins)
     for plugin in discovered_plugins:
-        logger.info("Found general plugin: %s", plugin.name)
         if allowed_plugins is None or plugin.name in allowed_plugins:
             try:
                 func = plugin.load()
                 func()
-                logger.info("Loaded general plugin: %s", plugin.name)
+                logger.info("plugin %s loaded.", plugin.name)
             except Exception:
-                logger.exception("Failed to load general plugin: %s",
-                                 plugin.name)
+                logger.exception("Failed to load plugin %s", plugin.name)
 
 
 _torch_compile_backend: Optional[Union[Callable, str]] = None