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 <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/file_windows.go

@@ -87,6 +87,8 @@ type dirInfo struct {
 func epipecheck(file *File, e error) {
 }
 
+// DevNull is the name of the operating system's ``null device.''
+// On Unix-like systems, it is "/dev/null"; on Windows, "NUL".
 const DevNull = "NUL"
 
 func (f *file) isdir() bool { return f != nil && f.dirinfo != nil }