doc: mention os.UserConfigDir in release notes

This change makes the release notes for Go 1.13 more complete
by mentioning a new function in the os package.

Change-Id: I0d637fd70ff6d14782bbfb7c13985a0f83b19d6d
Reviewed-on: https://go-review.googlesource.com/c/go/+/181945Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.13.html

@@ -265,6 +265,11 @@ TODO
 
 <dl id="os"><dt><a href="/pkg/os/">os</a></dt>
   <dd>
+    <p><!-- CL 160877 -->
+      The new <a href="/pkg/os/#UserConfigDir"><code>UserConfigDir</code></a> function
+      returns the default directory to use for user-specific configuration data.
+    </p>
+
     <p><!-- CL 166578 -->
       If a <a href="/pkg/os/#File"><code>File</code></a> is opened using the O_APPEND flag, its
       <a href="/pkg/os/#File.WriteAt"><code>WriteAt</code></a> method will always return an error.