Improve documentation on QT_LOGGING_DEBUG

I'm trying to debug an application, QGIS, that I know uses QtLogging. Some dependent libraries use QtLogging too. I can't figure out, how the logging is set up. 

Then I found (source: https://doc.qt.io/qt-5/qloggingcategory.html)

Set the QT_LOGGING_DEBUG environment variable to find out where your logging rules are loaded from.

So I tried:

$ QT_LOGGING_DEBUG=1 qgis

And I expected that something would be printed to the console about the logging rules. But that did not happen. 

Did I misunderstand what it's supposed to do? Could you please explain exactly how to use this feature. 

Sincerely, Thomas