--- a/xml/en/docs/njs/reference.xml
+++ b/xml/en/docs/njs/reference.xml
@@ -9,7 +9,7 @@
 <article name="Reference"
         link="/en/docs/njs/reference.html"
         lang="en"
-        rev="89">
+        rev="90">
 
 <section id="summary">
 
@@ -173,6 +173,26 @@ can be accessed with the syntax:
 </para>
 
 <para>
+Outgoing headers should be set before a response header is sent to a client,
+otherwise header update will be ignored.
+This means that <literal>r.headersOut{}</literal> is effectively writable in:
+
+<list type="bullet">
+
+<listitem>
+the <link doc="../http/ngx_http_js_module.xml" id="js_content"/> handler before
+<link id="r_send_header"><literal>r.sendHeader()</literal></link> or
+<link id="r_return"><literal>r.return()</literal></link> are called
+</listitem>
+
+<listitem>
+the <link doc="../http/ngx_http_js_module.xml" id="js_header_filter"/> handler
+</listitem>
+
+</list>
+</para>
+
+<para>
 Field values of multi-value response headers
 (<link doc="changes.xml" id="njs0.4.0">0.4.0</link>)
 can be set with the syntax: