This change simplifies the documentation on methods of UDPConn and
adds a reference to golang.org/x/net/{ipv4,ipv6} packages to the
documentation on {Read,Write}MsgUDP methods.

Change-Id: I425a8d81bc46b6579aa9f89faa4982bb86b40f24
Reviewed-on: https://go-review.googlesource.com/44912
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>