Fix for issue #3526620.
diff --git a/expat/doc/reference.html b/expat/doc/reference.html
index 8811a33..a4ab405 100644
--- a/expat/doc/reference.html
+++ b/expat/doc/reference.html
@@ -2151,8 +2151,12 @@
function behavior. In order to have an effect this must be called
before parsing has started. Returns 1 if successful, 0 when called
after <code>XML_Parse</code> or <code>XML_ParseBuffer</code>.
-<p><b>Note:</b> This call is optional, as the parser will auto-generate a new
-random salt value if no value has been set at the start of parsing.</p>
+<p><b>Note:</b>This call is optional, as the parser will auto-generate
+a new random salt value if no value has been set at the start of parsing.
+<p><b>Note:</b>One should not call <code>XML_SetHashSalt</code> with a
+hash salt value of 0, as this value is used as sentinel value to indicate
+that <code>XML_SetHashSalt</code> has <b>not</b> been called. Consequently
+such a call will have no effect, even if it returns 1.</p>
</div>
<pre class="fcndec" id="XML_UseForeignDTD">
