doc: update http server response 'close' event

Two places emits `close` events:

`resOnFinish` calls `emitCloseNT`, which happens when the result is
completed:
https://github.com/nodejs/node/blob/687dbd85263f433cc351c6daa83f9296a1d0bb4f/lib/_http_server.js#L695

It gets attached once the socket is assigned:
https://github.com/nodejs/node/blob/687dbd85263f433cc351c6daa83f9296a1d0bb4f/lib/_http_server.js#L226

And detached if the response is completed:
https://github.com/nodejs/node/blob/master/lib/_http_server.js#L709
https://github.com/nodejs/node/blob/687dbd85263f433cc351c6daa83f9296a1d0bb4f/lib/_http_server.js#L232

Which means, it can only be reached in this case, for premature socket
termination.

PR-URL: #34472
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/http.md

@@ -1239,7 +1239,8 @@ passed as the second parameter to the [`'request'`][] event.
 added: v0.6.7
 -->
 
-Indicates that the underlying connection was terminated.
+Indicates that the the response is completed, or its underlying connection was
+terminated prematurely (before the response completion).
 
 ### Event: `'finish'`
 <!-- YAML