doc: add release notes for log

Change-Id: Ifd330053094c172b7dcd9086da173289efc7f9b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/181942Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/go1.13.html

@@ -192,7 +192,7 @@ TODO
 <dl id="log"><dt><a href="/pkg/log/">log</a></dt>
   <dd>
     <p><!-- CL 168920 -->
-      TODO: <a href="https://golang.org/cl/168920">https://golang.org/cl/168920</a>: expose Writer() method of the standard logger
+      The new <a href="/pkg/log/#Writer"><code>Writer</code></a> function returns the output destination for the standard logger.
     </p>
 
 </dl><!-- log -->