Fix union type tagging for xdiff_string_rabdiff()

Follow up of 1987020

Author: Girgias

reference/xdiff/functions/xdiff-string-rabdiff.xml

@@ -11,7 +11,7 @@
  <refsect1 role="description">
   &reftitle.description;
   <methodsynopsis>
-   <type>string|false</type><methodname>xdiff_string_rabdiff</methodname>
+   <type class="union"><type>string</type><type>false</type></type><methodname>xdiff_string_rabdiff</methodname>
    <methodparam><type>string</type><parameter>old_data</parameter></methodparam>
    <methodparam><type>string</type><parameter>new_data</parameter></methodparam>
   </methodsynopsis>