stream: change comment on duplex stream options

'options.XXX' did not convey to me how to use camelCase for different
readable and writable options on a duplex stream. Give an example
instead.

PR-URL: #24247
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/_stream_writable.js

@@ -55,8 +55,8 @@ function WritableState(options, stream, isDuplex) {
   // Duplex streams are both readable and writable, but share
   // the same options object.
   // However, some cases require setting options to different
-  // values for the readable and the writable sides of the duplex stream.
-  // These options can be provided separately as readableXXX and writableXXX.
+  // values for the readable and the writable sides of the duplex stream,
+  // e.g. options.readableObjectMode vs. options.writableObjectMode, etc.
   if (typeof isDuplex !== 'boolean')
     isDuplex = stream instanceof Stream.Duplex;