[doc] Add a note to HostnameVerification about AHC 1.8.x bug

After having several people run into https://github.com/playframework/playframework/issues/2767 aka https://github.com/AsyncHttpClient/async-http-client/issues/555 (lately http://onelineadayblog.wordpress.com/2014/07/23/play-framework-2-3-ssl-hostname-verification/) I feel it's worthwhile to have a note explicitly in the docs until 1.9.x comes out.