doc: remove old note, 'cluster' is marked stable

Fixes: nodejs#3313
PR-URL: nodejs#3314
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/cluster.markdown

@@ -39,10 +39,7 @@ Running Node.js will now share port 8000 between the workers:
     23521,Master Worker 23523 online
     23521,Master Worker 23528 online
 
-This feature was introduced recently, and may change in future versions.
-Please try it out and provide feedback.
-
-Also note that, on Windows, it is not yet possible to set up a named pipe
+Please note that, on Windows, it is not yet possible to set up a named pipe
 server in a worker.
 
 ## How It Works