Document the new 'trace' feature.

Author: SergioBenitez

core/lib/src/lib.rs

@@ -53,20 +53,21 @@
 //!
 //! ## Features
 //!
-//! To avoid compiling unused dependencies, Rocket gates certain features. With
-//! the exception of `http2`, all are disabled by default:
+//! To avoid compiling unused dependencies, Rocket feature-gates optional
+//! functionality, some enabled by default:
 //!
-//! | Feature         | Description                                             |
-//! |-----------------|---------------------------------------------------------|
-//! | `secrets`       | Support for authenticated, encrypted [private cookies]. |
-//! | `tls`           | Support for [TLS] encrypted connections.                |
-//! | `mtls`          | Support for verified clients via [mutual TLS].          |
-//! | `http2`         | Support for HTTP/2 (enabled by default).                |
-//! | `json`          | Support for [JSON (de)serialization].                   |
-//! | `msgpack`       | Support for [MessagePack (de)serialization].            |
-//! | `uuid`          | Support for [UUID value parsing and (de)serialization]. |
-//! | `tokio-macros`  | Enables the `macros` feature in the exported `tokio`    |
-//! | `http3-preview` | Experimental preview support for [HTTP/3].              |
+//! | Feature         | Default? | Description                                             |
+//! |-----------------|----------|---------------------------------------------------------|
+//! | `trace`         | Yes      | Enables the default Rocket tracing [subscriber].        |
+//! | `http2`         | Yes      | Support for HTTP/2 (enabled by default).                |
+//! | `secrets`       | No       | Support for authenticated, encrypted [private cookies]. |
+//! | `tls`           | No       | Support for [TLS] encrypted connections.                |
+//! | `mtls`          | No       | Support for verified clients via [mutual TLS].          |
+//! | `json`          | No       | Support for [JSON (de)serialization].                   |
+//! | `msgpack`       | No       | Support for [MessagePack (de)serialization].            |
+//! | `uuid`          | No       | Support for [UUID value parsing and (de)serialization]. |
+//! | `tokio-macros`  | No       | Enables the `macros` feature in the exported `tokio`    |
+//! | `http3-preview` | No       | Experimental preview support for [HTTP/3].              |
 //!
 //! Disabled features can be selectively enabled in `Cargo.toml`:
 //!
@@ -82,6 +83,7 @@
 //! rocket = { version = "0.6.0-dev", default-features = false }
 //! ```
 //!
+//! [subscriber]: crate::trace::subscriber
 //! [JSON (de)serialization]: crate::serde::json
 //! [MessagePack (de)serialization]: crate::serde::msgpack
 //! [UUID value parsing and (de)serialization]: crate::serde::uuid