doc/go1.16: mention os.DirFS in os section

For #40700
For #41190

Change-Id: I8ade6efd5be09003fc3e5db5a9b91ba6e0f023f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/285593
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/go1.16.html

@@ -982,6 +982,13 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       <a href="/pkg/io/fs/"><code>io/fs</code></a> package.
       This should not affect any existing code.
     </p>
+
+    <p><!-- CL 243911 -->
+      The new <a href="/pkg/os/#DirFS"><code>DirFS</code></a> function
+      provides an implementation of
+      <a href="/pkg/io/fs/#FS"><code>fs.FS</code></a> backed by a tree
+      of operating system files.
+    </p>
   </dd>
 </dl><!-- os -->