Also updates package documentation for AIX.

Change-Id: I4809a334f0a4b8f3fb5345749477394b8f3e7546
Reviewed-on: https://go-review.googlesource.com/c/147678
Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>