Log Files

Is there a description of log files in /var/log ?

most files there are written by the system's syslog-deamon (in debian's case (on which PVE/PMG are based) - rsyslog).

There is quite much information on syslog on the net - e.g.:
* http://man7.org/linux/man-pages/man3/syslog.3.html
* https://www.thegeekdiary.com/linux-os-service-syslog/
*
https://wiki.debian.org/Rsyslog
The config is in /etc/rsyslog.conf and /etc/rsyslog.d/*

In many current Linux distros (including PVE/PMG) logging happens first with systemd-journald (you can read the journal with journalctl) and is then handed by systemd-journald to the syslog-process.

I hope this helps!