doc/go1.20: add a release note for os/exec API changes

Updates #50436.

Change-Id: Ib6771221bda1c81d5593b29d7287ebcf169882ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/449076
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

doc/go1.20.html

@@ -270,6 +270,17 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   </dd>
 </dl><!-- net/http/httputil -->
 
+<dl id="os/exec"><dt><a href="/pkg/os/exec/">os/exec</a></dt>
+  <dd>
+    <p><!-- https://go.dev/issue/50436, CL 401835 -->
+      The new <code>Cmd</code> fields <code>Cancel</code> and <code>WaitDelay</code>
+      specify the behavior of the <code>Cmd</code> when its associated
+      <code>Context</code> is canceled or its process exits with I/O pipes still
+      held open by a child process.
+    </p>
+  </dd>
+</dl><!-- os/exec -->
+
 <dl id="strconv"><dt><a href="/pkg/strconv/">strconv</a></dt>
   <dd>
     <p><!-- CL 345488 -->