logging categories are undocumented

The doc page QLoggingCategory does not adequately explain the concept of categories.

It says that they are "identified by a string." It explains that one may append ".debug", ".info", ".warning" or ".critical" to that string. It gives a few example strings, namely "driver.usb", "driver.usb.events", and "*". This is not enough to make productive use of logging categories.

As an API user, I would like to know:

• what Qt defined categories do exist
• how infer from a category string which classes generate messages in this category
• how to infer from a class documentation which categories are used for messages generated by this class
• how to set categories for messages generated by my own code

Disclosure: this is a thoroughly rewritten resubmission of closed report 66051.