Hibernate.cfg.xml specification

Where can I find hibernate.cfg.xml specification?

You can find the XML definition file at this URL: https://hibernate.org/dtd/hibernate-configuration-3.0.dtd.

Maybe I should ask for a reference instead of a specification.
I need to know some details of mappings not just the xml structure.

You mean a reference of the configuration settings? You can find it in Appendix A: Configuration Settings here: Hibernate ORM User Guide.