8234756: (dc spec) DatagramChannel.read should not specify that they return -1

[Brian Burkhalter]

Please review this simple doc change [1] to fix [2]. CSR will follow code review.

Thanks,

Brian

[1] http://cr.openjdk.java.net/~bpb/8234756/webrev.00/
[2] https://bugs.openjdk.java.net/browse/JDK-8234756