How to get file descriptor from NetConn for EPOLL handling?

Hi, as titled. We are trying to get the raw TCP connection to read the file descriptor.

```go
netConn := websocket.NetConn(reqCtx.Context(), wsConn, websocket.MessageText)
```

We want to use this library in combination with https://github.com/smallnest/epoller

Start by reading the websocket.NetConn usage shown in the issue and the library's NetConn documentation or implementation, then review the linked smallnest/epoller integration requirements. Determine whether raw file-descriptor access is supported or requires a documented API change; done means the supported approach or limitation is clearly documented.

Written by the indexing model from the issue text.