DevNull is documented on darwin, dragonfly, freebsd, linux,
nacl, netbsd, openbsd, solaris and plan9, but not on windows.
Add missing documentation.

Change-Id: Icdbded0dd5e322ed4360cbce6bee4cdca5cfbe72
Reviewed-on: https://go-review.googlesource.com/102456
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>