Also updates comment on isConnected field of netFD for clarification.

Change-Id: Icb1b0332e3b4c7802eae00ddc26cd5ba54c82dc2
Reviewed-on: https://go-review.googlesource.com/120955
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>