Document custom logging format better

Kea uses log4cplus library for logging. There are multiple formatting options available, but they're not well documented.

We should:

1. fix the link in the ARM Section 19.1.2
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