This change simplifies the documentation on methods of IPConn and adds
a reference to golang.org/x/net/ipv{4,6} packages to the documentation
on {Read,Write}MsgIP methods.

Change-Id: Ie07a853288940e0fef6a417ffc8d0c3d444c21cd
Reviewed-on: https://go-review.googlesource.com/44911
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>