Where to get additional information about php-fpm log messages?

Shortly: where can I get additional information about php-fpm log messages?

Details

I'm encountering some php-fpm issues, and with normal log level almost nothing is written in the log. So I set log level to debug to try and get additional informations.

Unfortunately some of the log entries are not really self-explaining. I searched extensively on Google and PHP.net site for documentation or reference but could not find any.

Is there an official or unofficial main resource about explaining php-fpm debug log messages?

Thank you in advance for any suggestion.