Document custom logging format better
Kea uses log4cplus library for logging. There are multiple formatting options available, but they're not well documented.
- fix the link in the ARM Section 19.1.2
- document available formatting options in Kea directly, rather than provide link to external docs (that we know broke down in the past)
In particular, we should document
%Q, which is microsecond (log4cplus calls it fractional milliseconds) timestamp.
The data is available in log4cplus sources: https://github.com/log4cplus/log4cplus/blob/master/include/log4cplus/layout.h#L293