fix: forgot one explanation

Author: jessekoconnor

doc/api/http.md

@@ -505,6 +505,10 @@ event is not being listened for and the response status code is 101 Switching
 Protocols, clients receiving an upgrade header will have their connections
 closed.
 
+This event is guaranteed to be passed an instance of the {net.Socket} class,
+a subclass of Duplex, instead of Duplex unless the user specifies a socket
+type other than {net.Socket}.
+
 A client server pair demonstrating how to listen for the `'upgrade'` event.
 
 ```js