Improve log formats documentation

See MDEV-19729.
Log formats are loosely defined in the documentation. There are no strict definition for file log formats, which makes developing a parser, a job of back engineering the log and testing it agains self generated log messages, instead of just implementing a spec. I think the documentation should include log format specification, like this:
Error log:

(date:<yyyy-MM-dd hh:mm:ss|yyyyMMdd hh:mm:ss>) <[(logger)]> (message)