Make the documentation clear about this.
https://bugzilla.gnome.org/show_bug.cgi?id=679671
*
* Parses @xml_data and returns a #GDBusNodeInfo representing the data.
*
+ * The introspection XML must contain exactly one top-level
+ * <tag class="starttag">node</tag> element.
+ *
* Note that this routine is using a
* <link linkend="glib-Simple-XML-Subset-Parser.description">GMarkup</link>-based
* parser that only accepts a subset of valid XML documents.