doc/go1.7.html: mention Server.Serve HTTP/2 behavior change

Fixes #16550
Updates #15908

Change-Id: Ic951080dbc88f96e4c00cdb3ffe24a5c03079efd
Reviewed-on: https://go-review.googlesource.com/25389Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/go1.7.html

@@ -916,6 +916,12 @@ For example, the address on which a request received is
 <code>req.Context().Value(http.LocalAddrContextKey).(net.Addr)</code>.
 </p>
 
+<p>
+The server's <a href="/pkg/net/http/#Server.Serve"><code>Serve</code></a> method
+now only enables HTTP/2 support if the <code>Server.TLSConfig</code> field is <code>nil</code>
+or includes <code>"h2"</code> in its <code>TLSConfig.NextProto</code>.
+</p>
+
 <p>
 The server implementation now
 pads response codes less than 100 to three digits