Merge pull request #4787 from coreos/xiang90-patch-1

doc: remove exp note for auth api

Documentation/authentication.md

@@ -1,7 +1,5 @@
 # Authentication Guide
 
-**NOTE: The authentication feature is considered experimental. We may change workflow without warning in future releases.**
-
 ## Overview
 
 Authentication -- having users and roles in etcd -- was added in etcd 2.1. This guide will help you set up basic authentication in etcd.