doc: mention `warn` and `error` as available log levels

For some users, this might be their first time being interacting with
the `env_logger` crate.
As such, they might not be aware that less verbose log levels exist.
Some might not want to log every incoming request, for example.

This commit also adds syntax hints to the code-fence for bash for better
syntax highlighting of that section, and repeats itself multiple times,
that `info` is, in fact, the default.

No changes to the recommendation of log levels were made.

0 files changed, 0 insertions, 0 deletions