doc/go1.21: document new bytes.Buffer methods

For #53685

Change-Id: I237297d19afeb36ad738074d0c61caa7012f65ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/499617
Reviewed-by: Eli Bendersky <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Bypass: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

doc/go1.21.html

@@ -333,12 +333,14 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 
 <dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
   <dd>
-    <p><!-- https://go.dev/issue/53685 -->
-      TODO: <a href="https://go.dev/issue/53685">https://go.dev/issue/53685</a>: add Buffer.Available and Buffer.AvailableBuffer
-    </p>
-
-    <p><!-- CL 474635 -->
-      TODO: <a href="https://go.dev/cl/474635">https://go.dev/cl/474635</a>: bytes: add Buffer.Available and Buffer.AvailableBuffer; modified api/next/53685.txt
+    <p><!-- https://go.dev/issue/53685, CL 474635 -->
+      The <a href="/pkg/bytes/#Buffer"><code>Buffer</code></a> type
+      has two new methods:
+      <a href="/pkg/bytes/#Buffer.Available"><code>Available</code></a>
+      and <a href="/pkg/bytes/#AvailableBuffer"><code>AvailableBuffer</code></a>.
+      These may be used along with the
+      <a href="/pkg/bytes/#Buffer.Write"><code>Write</code></a>
+      method to append directly to the <code>Buffer</code>.
     </p>
   </dd>
 </dl><!-- bytes -->