doc/go1.20: mention that calling T.Run from T.Cleanup will panic

For golang/go#48515

Change-Id: I1c616144a58e92fe4022d0e86f4208d68dcce816
Reviewed-on: https://go-review.googlesource.com/c/website/+/475215
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

_content/doc/go1.20.html

@@ -1192,6 +1192,13 @@ proxyHandler := &httputil.ReverseProxy{
       reports the current elapsed time of the benchmark, which may be useful for
       calculating rates to report with <code>ReportMetric</code>.
     </p>
+
+    <p><!-- https://go.dev/issue/48515, CL 352349 -->
+      Calling <a href="/pkg/testing/#T.Run"><code>T.Run</code></a>
+      from a function passed
+      to <a href="/pkg/testing/#T.Cleanup"><code>T.Cleanup</code></a>
+      was never well-defined, and will now panic.
+    </p>
   </dd>
 </dl><!-- testing -->