doc: clarify http timeouts

Socket timeouts set using the `'connection'` event are replaced by
`server.keepAliveTimeout` when a response is handled.

PR-URL: #25748
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

doc/api/http.md

@@ -908,6 +908,10 @@ also be accessed at `request.connection`.
 This event can also be explicitly emitted by users to inject connections
 into the HTTP server. In that case, any [`Duplex`][] stream can be passed.
 
+If `socket.setTimeout()` is called here, the timeout will be replaced with
+`server.keepAliveTimeout` when the socket has served a request (if
+`server.keepAliveTimeout` is non-zero).
+
 ### Event: 'request'
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