doc: update parking_lot info in feature list (#5215)

tokio/src/lib.rs

@@ -328,20 +328,15 @@
 //! - `signal`: Enables all `tokio::signal` types.
 //! - `fs`: Enables `tokio::fs` types.
 //! - `test-util`: Enables testing based infrastructure for the Tokio runtime.
+//! - `parking_lot`: As a potential optimization, use the _parking_lot_ crate's
+//!                  synchronization primitives internally. Also, this
+//!                  dependency is necessary to construct some of our primitives
+//!                  in a const context. MSRV may increase according to the
+//!                  _parking_lot_ release in use.
 //!
 //! _Note: `AsyncRead` and `AsyncWrite` traits do not require any features and are
 //! always available._
 //!
-//! ### Internal features
-//!
-//! These features do not expose any new API, but influence internal
-//! implementation aspects of Tokio, and can pull in additional
-//! dependencies.
-//!
-//! - `parking_lot`: As a potential optimization, use the _parking_lot_ crate's
-//! synchronization primitives internally. MSRV may increase according to the
-//! _parking_lot_ release in use.
-//!
 //! ### Unstable features
 //!
 //! Some feature flags are only available when specifying the `tokio_unstable` flag: