doc: undo move http.IncomingMessage.statusMessage

The description for `statusMessage` was accidentally moved under the
heading for `http.IncomingMessage.socket`.  This commit puts it back
in the correct place.

PR-URL: https://github.com/nodejs/node/pull/4822
Fixes: https://github.com/nodejs/node/issues/4558
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/http.markdown

@@ -939,6 +939,8 @@ The 3-digit HTTP response status code. E.G. `404`.
 
 **Only valid for response obtained from [`http.ClientRequest`][].**
 
+The HTTP response status message (reason phrase). E.G. `OK` or `Internal Server Error`.
+
 ### message.socket
 
 The [`net.Socket`][] object associated with the connection.
@@ -946,8 +948,6 @@ The [`net.Socket`][] object associated with the connection.
 With HTTPS support, use [`request.socket.getPeerCertificate()`][] to obtain the
 client's authentication details.
 
-The HTTP response status message (reason phrase). E.G. `OK` or `Internal Server Error`.
-
 ### message.trailers
 
 The request/response trailers object. Only populated at the `'end'` event.