subscriber: correct fmt::init() documentation (#2224)

## Motivation

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`.

## Solution

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 #2217
Fixes #1329

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

tracing-subscriber/src/fmt/mod.rs

@@ -1203,13 +1203,19 @@ pub fn try_init() -> Result<(), Box<dyn Error + Send + Sync + 'static>> {
 /// Install a global tracing subscriber that listens for events and
 /// filters based on the value of the [`RUST_LOG` environment variable].
 ///
+/// The configuration of the subscriber initialized by this function
+/// depends on what [feature flags](crate#feature-flags) are enabled.
+///
 /// If the `tracing-log` feature is enabled, this will also install
 /// the LogTracer to convert `Log` records into `tracing` `Event`s.
 ///
-/// This is shorthand for
+/// If the `env-filter` feature is enabled, this is shorthand for
 ///
 /// ```rust
-/// tracing_subscriber::fmt().init()
+/// # use tracing_subscriber::EnvFilter;
+/// tracing_subscriber::fmt()
+///     .with_env_filter(EnvFilter::from_default_env())
+///     .init();
 /// ```
 ///
 /// # Panics