[css-values] Update DoC

Author: fantasai

css-values/issues-cr-2015.bsi

@@ -164,7 +164,8 @@ Issue 20.
 Summary:  Define case-sensitivity of attr()
 From:     Simon Pieters
 Comment:  https://lists.w3.org/Archives/Public/www-style/2016Apr/0067.html
-Open
+Response: https://lists.w3.org/Archives/Public/www-style/2016Jun/0032.html
+Closed:   Accepted
 ----
 Issue 21.
 Summary:  Clarify <custom-ident> restrictions.