docs(trace): move trace documentation to lib and reduce information

Remove `trace` documetationfrom `trace.rs`. Add unstable section to
`lib.rs` detailing unstable features. Add information about `tracing`
and `ffi` to unstable features section.

src/lib.rs

@@ -51,7 +51,20 @@
 //! - `server`: Enables the HTTP `server`.
 //!
 //! [feature flags]: https://doc.rust-lang.org/cargo/reference/manifest.html#the-features-section
-
+//!
+//! # Unstable Features
+//! hyper includes a set of unstable optional features that can be enabled through the use of a
+//! feature flag and a [configuration flag].
+//!
+//! The following is a list of feature flags and their corresponding `RUSTFLAG`:
+//! - `ffi`: Enables C API for hyper `hyper_unstable_ffi`.
+//! - `tracing`: Enables debug logging with `hyper_unstable_tracing`.
+//!
+//! Enabling an unstable feature is possible with the following `cargo` command, as of version `1.64.0`:
+//! ```notrust
+//! RUSTFLAGS="--cfg hyper_unstable_tracing" cargo rustc --features client,http1,http2,tracing --crate-type cdylib
+//!```
+//! [configuration flag]: https://doc.rust-lang.org/reference/conditional-compilation.html
 #[doc(hidden)]
 pub use http;
 

src/trace.rs

@@ -2,33 +2,6 @@
 // even if they are not used at the present time.
 #![allow(unused_macros)]
 
-//! Internal Tracing macro module
-//!
-//! The [`trace`][crate::trace] module is an internal module that contains wrapper macros encapsulating
-//! [`tracing`][tracing]'s macros. These macros allow for conditional expansion of
-//! [`tracing`][tracing] macros during compilation when the `tracing` feature is enabled, or trimming
-//! them when the feature is disabled, all in a concise manner.
-//!
-//! The macros from the [`trace`][crate::trace] module are declared by default and can be used throughout the
-//! crate. However, as the contents of these macros are conditionally compiled, they are effectively trimmed
-//! during inline expansion when the `tracing` feature is disabled.
-//!
-//! # Unstable
-//!
-//! The [`tracing`][tracing] module is currenty **unstable**, hence the existence of this module. As a
-//! result, hyper's [`tracing`][tracing] logs are only accessibe if `--cfg hyper_unstable_tracing` is
-//! passed to `rustc` when compiling. The easiest way to do that is through setting the `RUSTFLAGS`
-//! enviornment variable.
-//!
-//! # Building
-//!  
-//! Enabling [`trace`][crate::trace] logs, can be done with the following `cargo` command, as of
-//! version `1.64.0`:
-//!
-//! ```notrust
-//! RUSTFLAGS="--cfg hyper_unstable_tracing" cargo rustc --features client,http1,http2,tracing --crate-type cdylib
-//! ```
-
 #[cfg(not(hyper_unstable_tracing))]
 compile_error!(
     "\