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>