doc/go1.18: add AppendRune doc

For #47694

Change-Id: I39594c273aeb038702457587ee1c46e4b3920bb6
Reviewed-on: https://go-review.googlesource.com/c/go/+/363359
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Trust: Jeremy Faller <jeremy@golang.org>

doc/go1.18.html

@@ -441,7 +441,8 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 <dl id="unicode/utf8"><dt><a href="/pkg/unicode/utf8/">unicode/utf8</a></dt>
   <dd>
     <p><!-- CL 345571 -->
-      TODO: <a href="https://golang.org/cl/345571">https://golang.org/cl/345571</a>: add AppendRune
+      The <a href="/pkg/unicode/utf8/#AppendRune"><code>AppendRune</code></a> function appends the UTF-8 new
+      encoding of a <code>rune</code> to a <code>[]byte</code>.
     </p>
   </dd>
 </dl><!-- unicode/utf8 -->