_content/doc/go1.20: add release note for new cookie.valid behavior

Fixes #58485

Add a release note documenting that Cookie.Valid ignores the
Cookie.Expires field when empty.

Change-Id: Ia0138dc2252c309c36ff774e30e4aa9a874bf1e5
GitHub-Last-Rev: 6355670ad068dc451c5fd2bed1ea6f2a1fa4016c
GitHub-Pull-Request: golang/website#193
Reviewed-on: https://go-review.googlesource.com/c/website/+/468716
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
diff --git a/_content/doc/go1.20.html b/_content/doc/go1.20.html
index 2bae0c6..358e84a 100644
--- a/_content/doc/go1.20.html
+++ b/_content/doc/go1.20.html
@@ -973,6 +973,11 @@
       For example, a cookie setting of "name =value"
       is now accepted as setting the cookie "name".
     </p>
+
+    <p><!-- https://go.dev/issue/52989 -->
+     A <a href="/pkg/net/http#Cookie"><code>Cookie</code></a> with an empty Expires field is now considered valid.
+     <a href="/pkg/net/http#Cookie.Valid"><code>Cookie.Valid</code></a> only checks Expires when it is set.
+    </p>
   </dd>
 </dl><!-- net/http -->