I think it's preferable to use the named constants rather than string
literals for levels since it makes it easier to see what the options are
and catch typos in a more normal way.

Also document that the `level` parameter can be named constant, string
literal or numeric constant.

The doc references to `Level` don't resolve because `tracing-attribute`
doesn't have a direct dependency on `tracing` - not sure how to resolve
right now.

I hope there's a better way to deal with this.