RFR 8201317: X25519/X448 code improvements (xs)

[Adam Petcher]

I received some suggestions for improvements to the X25519/X448 code 
after the completion of the code review back in March. The improvements 
are some additional comments on methods and some rearranged expressions 
to prevent integer overflow. The change is very small, please review if 
you have 10 minutes to spare.

JBS: https://bugs.openjdk.java.net/browse/JDK-8201317
Webrev: http://cr.openjdk.java.net/~apetcher/8201317/webrev.00/