_content/doc/go1.8: re-add reserved GOOS=zos value mention

For golang/go#64169.

Change-Id: Iac58322c3bdd20a86e5492e5cdbb8e92182962a6
Reviewed-on: https://go-review.googlesource.com/c/website/+/542615
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

_content/doc/go1.8.md

@@ -87,6 +87,10 @@ and if the program terminates with output "ARMv6K supported." then the system
 implements ARMv6K or later.
 Go on non-Linux ARM systems already requires ARMv6K or later.
 
+<!-- CL 31596, go.dev/issue/17528 -->
+`zos` is now a recognized value for `GOOS`,
+reserved for the z/OS operating system.
+
 ### Known Issues {#known_issues}
 
 There are some instabilities on FreeBSD and NetBSD that are known but not understood.