doc: added tracing to the feature flags section
//! - `signal`: Enables all `tokio::signal` types.
//! - `fs`: Enables `tokio::fs` types.
//! - `test-util`: Enables testing based infrastructure for the Tokio runtime.
+//! - `tracing`: Enables tracing events
//!
//! _Note: `AsyncRead` and `AsyncWrite` traits do not require any features and are
//! always available._
