Would be nice to have 'Debug mode' documented.
Maybe under https://www.zabbix.com/documentation/2.2/manual/web_interface as 9th section.

The documentation might for instance cover things like:

• how to enable/disable (possibly referencing User groups)
• how to access
• how to use, e.g. some notes about color schema, different blocks, etc.

A mention in https://www.zabbix.com/documentation/2.2/manual/appendix/performance_tuning pointing to this might possibly be beneficial too.