Support and clearly document a means to change the default log level

[ssokolow]

I often write projects in Python where my logging level should be warn or even info if the user doesn't specify anything.

Currently, the docs are unclear whether there's a way to set a logging level, but still allow it to be overridden by RUST_LOG, other than by checking for the RUST_LOG environment variable and setting it if it's absent before calling env_logger::init()... a solution which, I think you'll agree, looks like an ugly hack.

[KodrAus]

Hi @ssokolow We do actually support a better way to default the log level to something else if an environment variable isn't present using the Env type. In this example, we default the log level to trace if the variable isn't present.

You're right we don't do a great job of surfacing this, the docs could definitely be improved.