[socket.io] Doc issue

In the migration doc, it says some APIs are changed from sync to async, but the code sample here doesn't make much difference:

```js
io.on("connection", async (socket) => { await socket.join("room abc"); });
```

For this particular case it doesn't really matter whether you put await or not.

Locate the migration document referenced in the issue and review the socket.join code sample alongside the surrounding explanation of synchronous and asynchronous APIs. Clarify the example so its behavior and the relevance of await are accurate, then verify that the revised wording and sample communicate the intended migration guidance.

Written by the indexing model from the issue text.