Add explanation

Co-authored-by: Max Leske <250711+theseion@users.noreply.github.com>

src/request_body_processor/xml.cc

@@ -184,6 +184,7 @@ bool XML::init() {
         m_data.xml_parser_state                  = std::make_unique<XMLNodes>(m_transaction);
         m_data.xml_parser_state->node_depth      = 0;
         m_data.xml_parser_state->currval         = "";
+        // the XML will contain at least one node, which is the pseudo root node 'xml'
         m_data.xml_parser_state->currpath        = "xml.";
     }