RFR: JDK-8235352: java.lang.runtime.package-info should be annotated with @PreviewFeature

[Vicente Romero]

Please review this simple fix which is adding the @PreviewFeature and 
related doc to, new for records, file 
java/lang/runtime/package-info.java. Webrev is at [1] and JIRA issue at [2]

Thanks,
Vicente

[1] http://cr.openjdk.java.net/~vromero/8235352/webrev.00/
[2] https://bugs.openjdk.java.net/browse/JDK-8235352