Previously the documentation for `fmt::init()` was misleading. It stated
that it was shorthand for `fmt().init()`. This lead to confusion as
users would expect the same behavior from both. However `fmt::init()`
would, whether you used the env-filter feature or not, rely on RUST_LOG
to set the tracing level. `fmt().init()` does not do this and it must be
set with a specific configuration via `with_env_filter`.

The documentation has been updated to no longer state that it is 1:1
shorthand for the other. The documentation now specifically points out
that you must be using the `env-filter` feature and gives a correct
example to mimic the `fmt::init()` behavior using `fmt().init()`.

fixes: tokio-rs#2217 tokio-rs#1329