_content/doc/go1.21.html: add note about panic nil behavior

For golang/go#25448.

Change-Id: I775ad6d8d8952da8fa94c39c44a2420501b63b57
Reviewed-on: https://go-review.googlesource.com/c/website/+/517036
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

_content/doc/go1.21.html

@@ -143,6 +143,20 @@ Do not send CLs removing the interior tags from such phrases.
   For details about how to try that language change, see <a href="https://go.dev/wiki/LoopvarExperiment">the LoopvarExperiment wiki page</a>.
 </p>
 
+<!-- https://go.dev/issue/25448 -->
+<p>
+  Go 1.21 now defines that if a goroutine is panicking and recover was called directly by a deferred
+  function, the return value of recover is guaranteed not to be nil. To ensure this, calling panic
+  with a nil interface value (or an untyped nil) causes a run-time panic of type
+  <a href="/runtime/#PanicNilError"><code>*runtime.PanicNilError</code></a>.
+</p>
+<p>
+  To support programs written for older versions of Go, nil panics can be re-enabled by setting
+  <code>GODEBUG=panicnil=1</code>.
+  This setting is enabled automatically when compiling a program whose main package
+  is in a module with that declares <code>go</code> <code>1.20</code> or earlier.
+</p>
+
 <h2 id="tools">Tools</h2>
 <p>
   Go 1.21 adds improved support for backwards compatibility and forwards compatibility