doc: update http.md mention of socket

doc/api/http.md

@@ -471,9 +471,11 @@ once.
 added: v0.5.3
 -->
 
-* `socket` {net.Socket}
+* `duplex` {stream.Duplex}
 
-Emitted after a socket is assigned to this request.
+Emitted after a socket is assigned to this request. This event is guaranteed to
+be passed an instance of the {net.Socket} class, a subclass of duplex, unless
+the user specifies a socket type other than {net.Socket}.
 
 ### Event: 'timeout'
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