doc: specify that message.socket can be nulled

The `socket` property of the `IncomingMessage` object is nulled on the
server after calling `message.destroy()` and on the client after a
request completes and the socket is kept alive.

Fixes: nodejs#41011

PR-URL: nodejs#41014
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>

doc/api/http.md

@@ -2321,7 +2321,7 @@ client's authentication details.
 
 This property is guaranteed to be an instance of the {net.Socket} class,
 a subclass of {stream.Duplex}, unless the user specified a socket
-type other than {net.Socket}.
+type other than {net.Socket} or internally nulled.
 
 ### `message.statusCode`