No documentation for log format fields

[wsw70]

The documentation paragraph Configuring the Logging Format mentions that

There are two ways to configure formatting: you can either just change the format string or hook in a custom format function.

All the handlers that come with logbook and that log into a string use the StringFormatter by default. Their constructors accept a format string which sets the logbook.Handler.format_string attribute. You can override this attribute in which case a new string formatter is set: (...)

There is no mention of the fields which can be used in the formatting (similar to the LogRecord attributes in standard Python logging module)

[vmalloc]

It's actually very much the same - formatting uses Logbook's own LogRecord class (See https://github.com/getlogbook/logbook/blob/develop/logbook/base.py#L348)