doc/go1.21: context.Background and TODO may now appear equal

Fixes #60978

Change-Id: I3e4bd366dc30ac435698b8f17170695330034683
Reviewed-on: https://go-review.googlesource.com/c/go/+/505795
TryBot-Bypass: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Sameer Ajmani <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

doc/go1.21.html

@@ -473,6 +473,16 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       The new <a href="/pkg/context/#AfterFunc"><code>AfterFunc</code></a>
       function registers a function to run after a context has been cancelled.
     </p>
+
+    <p><!-- CL 455455 -->
+      An optimization means that the results of calling
+      <a href="/pkg/context/#Background"><code>Background</code></a>
+      and <a href="/pkg/context/#TODO"><code>TODO</code></a> and
+      converting them to a shared type can be considered equal.
+      In previous releases they were always different.  Comparing
+      <a href="/pkg/context/#Context"><code>Context</code></a> values
+      for equality has never been well-defined, so this is not
+      considered to be an incompatible change.
   </dd>
 </dl>