Fix GH-4582: Incorrect function signature for xattr_get (#4585)

Author: nielsdos

reference/xattr/functions/xattr-get.xml

@@ -11,7 +11,7 @@
  <refsect1 role="description">
   &reftitle.description;
   <methodsynopsis>
-   <type>string</type><methodname>xattr_get</methodname>
+   <type class="union"><type>string</type><type>false</type></type><methodname>xattr_get</methodname>
    <methodparam><type>string</type><parameter>filename</parameter></methodparam>
    <methodparam><type>string</type><parameter>name</parameter></methodparam>
    <methodparam choice="opt"><type>int</type><parameter>flags</parameter><initializer>0</initializer></methodparam>