_content/doc/go1.8: document reserved GOOS=zos value

Mention its existence retroactively for posterity, similarly to how the
reserved GOOS=hurd value was documented in the Go 1.12 release notes.

For golang/go#17528.
For golang/go#17929.

Change-Id: Ia574d856f1fca8b387c9f25530843cc874f525b7
Reviewed-on: https://go-review.googlesource.com/c/website/+/537916
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
diff --git a/_content/doc/go1.8.html b/_content/doc/go1.8.html
index fc5b180..7e6dbfa 100644
--- a/_content/doc/go1.8.html
+++ b/_content/doc/go1.8.html
@@ -108,6 +108,10 @@
   Go on non-Linux ARM systems already requires ARMv6K or later.
 </p>
 
+<p id="zos"><!-- CL 31596, go.dev/issue/17528 -->
+  <code>zos</code> is now a recognized value for <code>GOOS</code>,
+  reserved for the z/OS operating system.
+</p>
 
 <h3 id="known_issues">Known Issues</h3>