YARN-8892. YARN UI2 doc changes to update security status (verified under security environment). (Sunil G via wangda)

Change-Id: I8bc8622936861b8d6de3e42a0b75af86ad8a3961
(cherry picked from commit 538250db26ce0b261bb74053348cddfc2d65cf52)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-site/src/site/markdown/YarnUI2.md

@@ -52,4 +52,4 @@ Open your browser, go to `rm-address:8088/ui2` and try it!
 Notes
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-- This UI framework is not verified under security environment, please use with caution under security environment.
+This UI framework is verified under security environment as well.