doc: add release notes for new context functions

For #40221
For #56661
For #57928

Change-Id: Iaf7425bb26eeb9c23235d13c786d5bb572159481
Reviewed-on: https://go-review.googlesource.com/c/go/+/486535
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Sameer Ajmani <sameer@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

doc/go1.21.html

@@ -84,6 +84,27 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
  TODO: complete this section
 </p>
 
+<dl id="context"><dt><a href="/pkg/context/">sync</a></dt>
+ <dd>
+ <p><!-- https://go.dev/issue/40221, CL 479918 -->
+ The new <a href="/pkg/context/#WithoutCancel"><code>WithoutCancel</code></a>
+ function returns a copy of a context that is not canceled when the original
+ context is canceled.
+ </p>
+ <p><!-- https://go.dev/issue/56661, CL 449318 -->
+ The new <a href="/pkg/context/#WithDeadlineCause"><code>WithDeadlineCause</code></a>
+ and <a href="/pkg/context/#WithTimeoutCause"><code>WithTimeoutCause</code></a>
+ functions provide a way to set a context cancellation cause when a deadline or
+ timer expires. The cause may be retrieved with the
+ <a href="/pkg/context/#Cause"><code>Cause</code></a> function.
+ </p>
+ <p><!-- https://go.dev/issue/57928, CL 482695 -->
+ The new <a href="/pkg/context/#AfterFunc"><code>AfterFunc</code></a>
+ function registers a function to run after a context has been cancelled.
+ </p>
+ </dd>
+</dl>
+
 <dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
  <dd>
  <p><!-- https://go.dev/issue/56102, CL 451356 -->