os: document DevNull on windows
os: document DevNull on windows

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 <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cloudflare/go,golang/go,Cofyc/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,josharian/go,cloudflare/go,akutz/go,josharian/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,Cofyc/go,nawawi/go,josharian/go,nawawi/go,cloudflare/go,josharian/go,akutz/go,cloudflare/go,Cofyc/go,golang/go,Cofyc/go,akutz/go,golang/go,CAFxX/go,akutz/go,golang/go,Cofyc/go,Cofyc/go,akutz/go,golang/go,CAFxX/go,josharian/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,akutz/go,josharian/go,nawawi/go,cloudflare/go,Cofyc/go,josharian/go,akutz/go,akutz/go,CAFxX/go,nawawi/go,CAFxX/go,Cofyc/go,golang/go,josharian/go