线程模型文档问题，不含响应、响应

execution 只有请求消息派发到线程池，不含响应，响应和其它连接断开事件，心跳等消息，直接在 IO 线程上执行。

Locate the thread-model documentation in the dubbo-website repository and compare its description with the issue. Clarify that execution dispatches request messages to the thread pool, while responses, connection-close events, and heartbeat messages run directly on the I/O thread. Done when the documentation states this distinction accurately.

Written by the indexing model from the issue text.