chore: add comment to the PLUGIN_DIFY_INNER_API_KEY key (#15381)

docker/.env.example

@@ -959,6 +959,7 @@ PLUGIN_DEBUGGING_PORT=5003
 EXPOSE_PLUGIN_DEBUGGING_HOST=localhost
 EXPOSE_PLUGIN_DEBUGGING_PORT=5003
 
+# If this key is changed, DIFY_INNER_API_KEY in plugin_daemon service must also be updated or agent node will fail.
 PLUGIN_DIFY_INNER_API_KEY=QaHbTe77CtuXmsfyhR7+vRjI/+XbV1AaFy691iy+kGDv2Jvy0/eAh8Y1
 PLUGIN_DIFY_INNER_API_URL=http://api:5001