message: docs: update top-level documentation of tracing-subscriber features
diff: //! Enabled by default.
  //! - `ansi`: Enables `fmt` support for ANSI terminal colors. Enabled by
  //! default.
  -//! - `registry`: enables the experimental [`registry`] module.
  +//! - `registry`: enables the [`registry`] module. Enabled by default.
  //! - `json`: Enables `fmt` support for JSON output. In JSON output, the ANSI feature does nothing.
  //!
  //! ### Optional Dependencies
  