OK, I have check module code (on https://github.com/ansible-collections/community.general/blob/main/plugins/modules/xml.py#L847)
And at line 435, comment mentioned that lxml 5.1.1 has deprecated _ElementStringResult. If I downgrade lxml to 5.1.0, script is OK and document is change as I want !
Ansible module can no longer use lxml after version 5.1.0
I have open an issue on github for that. Thanks