unserialize(): fix XML tags for union type (#4404)

Author: nsfisis

reference/var/functions/unserialize.xml

@@ -89,7 +89,7 @@
         <tbody>
          <row>
           <entry><literal>allowed_classes</literal></entry>
-          <entry><type>array|bool</type></entry>
+          <entry><type class="union"><type>array</type><type>bool</type></type></entry>
           <entry>
            <simpara>
             Either an <type>array</type> of class names which should be