8198753: (dc) DatagramChannel throws unspecified exceptions

[Brian Burkhalter]

https://bugs.openjdk.java.net/browse/JDK-8198753
http://cr.openjdk.java.net/~bpb/8198753/webrev.00/

Change an undocumented IllegalArgumentException to AlreadyConnectedException and document some other undocumented exceptions.

CSR to follow.

Thanks,

Brian