diff --git a/doc/config.html b/doc/config.html
index 4707a182..1ed68b49 100755
--- a/doc/config.html
+++ b/doc/config.html
@@ -492,6 +492,13 @@ entry when creating a reply to that entry. This can make sense for example for
 the author, since the author of a reply can be different from the original author.
 <p>
 
+<LI><b><code>Subst on reply &lt;attribute &gt; = &lt;string&gt;</code></b>
+<br>
+Substitution of attributes for replies. This option can be used to replace the current
+subject with a "Re: &lt;old subject&gt;":<br>
+<pre>Subst on reply subject = Re: $subject</pre>
+<p>
+
 </UL>
 
 <a name="access"><hr>