diff --git a/doc/manual/expressions/builtins.xml b/doc/manual/expressions/builtins.xml
index 14da1c63e..5e88b8856 100644
--- a/doc/manual/expressions/builtins.xml
+++ b/doc/manual/expressions/builtins.xml
@@ -600,6 +600,16 @@ x: x + 456</programlisting>
   </varlistentry>
 
 
+  <varlistentry><term><function>builtins.isFloat</function>
+  <replaceable>e</replaceable></term>
+
+    <listitem><para>Return <literal>true</literal> if
+    <replaceable>e</replaceable> evaluates to a float, and
+    <literal>false</literal> otherwise.</para></listitem>
+
+  </varlistentry>
+
+
   <varlistentry><term><function>builtins.isBool</function>
   <replaceable>e</replaceable></term>