doc/go1.21: mention that embedded files implement ReadAt

For #57803

Change-Id: I8e33f4dd3fc3071bfbf4d2848faefbe8488f5742
Reviewed-on: https://go-review.googlesource.com/c/go/+/499418
TryBot-Bypass: Ian Lance Taylor <iant@google.com>
Reviewed-by: Eli Bendersky <eliben@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

doc/go1.21.html

@@ -462,12 +462,12 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 
 <dl id="embed"><dt><a href="/pkg/embed/">embed</a></dt>
   <dd>
-    <p><!-- https://go.dev/issue/57803 -->
-      TODO: <a href="https://go.dev/issue/57803">https://go.dev/issue/57803</a>: openFile should implement ReaderAt
-    </p>
-
-    <p><!-- CL 483235 -->
-      TODO: <a href="https://go.dev/cl/483235">https://go.dev/cl/483235</a>: embed: implement openFile.ReadAt
+    <p><!-- https://go.dev/issue/57803, CL 483235 -->
+      The <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
+      returned by
+      <a href="/pkg/embed/#FS.Open"><code>FS.Open</code></a> now
+      has a <code>ReadAt</code> method that
+      implements <a href="/pkg/io/#ReaderAt"><code>io.ReaderAt</code></a>.
     </p>
 
     <p><!-- https://go.dev/issue/54451, CL 491175 -->