This change simplifies the documentation on methods of UnixConn.

Change-Id: Ibd89f9172b28adfcf7fdd43b3bc196ae7f8302cf
Reviewed-on: https://go-review.googlesource.com/44913
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>