Improve docs of LoggingCategory QML Type

Documentation of LoggingCategory QML Type contains typos, minor incorrectnesses and only one little example. It was written by the same person who did the actual code, and while he seems to be good at coding, the documentation could use some skills from technical writing professionals.

I can give it a try, but I'm afraid it would require a review from someone like Paul anyway.

Todo list:

• rewording/typos
• more useful examples, with expected output (assuming default logging handler)
• documented LogLevel enum
• cross-link from QLoggingCategory Class page