RFR: JDK-8246098: API for Class::permittedSubclasses should clarify if returned elements are ordered or not

[Vicente Romero]

Please review this small change to the API for 
java.lang.Class::permittedSubclasses which clarifies that no particular 
order of the returned elements should be assumed by clients. The related 
bug is at [1], the webrev at [2] and the associated CSR at [3],

Thanks,
Vicente
[1] https://bugs.openjdk.java.net/browse/JDK-8246098
[2] http://cr.openjdk.java.net/~vromero/8246098/webrev.00/
[3] https://bugs.openjdk.java.net/browse/JDK-8247361