doc: mention net/http Server Content-Type change

Updates #20784

Change-Id: Ic07c78a86da5026e407ac9ecb3117d320c198048
Reviewed-on: https://go-review.googlesource.com/82016Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/go1.10.html

@@ -1214,6 +1214,10 @@ The content-serving handlers also now omit the <code>Content-Type</code> header
 if passed an invalid (non-3-digit) status code.
 </p>
 <p>
+<!-- CL 46631 -->
+The <code>Server</code> will no longer add an implicit Content-Type when a <code>Handler</code> does not write any output.
+</p>
+<p>
 <a href="/pkg/net/http/#Redirect"><code>Redirect</code></a> now sets the <code>Content-Type</code> header before writing its HTTP response.
 </p>
 </dl>