I've just posted a patch "mantisbt-1.2.3_documentation_time-tracking.patch" that add documentation about Time Tracking in sections 'admin.config.timetracking' and 'admin.project.timetracking'.
It's a simple copy/paste from the Wiki (http://www.mantisbt.org/wiki/doku.php/mantisbt:issue:4428).

The patch is on the bugtracker : http://www.mantisbt.org/bugs/view.php?id=12566

Becarreful: I do not know how to run the documentation Makefile, so it's a raw redaction without a build. I did very very carrefully, but it could be some sgml error inside.