RFR: JDK-8192935 Fix EnumSet's SerializationProxy javadoc

[Martin Buchholz]

   1. JDK-8192935 <https://bugs.openjdk.java.net/browse/JDK-8192935>

http://cr.openjdk.java.net/~martin/webrevs/openjdk10/EnumSet-SerializationProxy/EnumSet-SerializationProxy.patch