Improve documentation of LogBackend configuration

[straight-shoota]

The readme and API docs show which options exist to configure LogBackend. But it's not clear what they mean or what the default behaviour is.

From reading the code I assume that by default LogBackend doesn't actually log anything. You need to set capture_exceptions: true to capture log entries with exceptions or capture_all: true to capture every log entry.

I presume both settings together don't really make sense? When capture_all: true the value of capture_exceptions doesn't matter. So maybe it would be better to unify those configuration values to a single option with different levels (as an enum)? Also - if my assumption about not logging anything by default is correct, wouldn't it be more sensible to log at least exceptions if you don't configure anything? What else would be the point of hooking up a LogBackend?