docs: document tracing-subscriber valuable support (tokio-rs#1901)

This PR adds documentation on `valuable` support in `tracing-serde` and
`tracing-subscriber`.

tracing-serde/src/lib.rs

@@ -375,6 +375,7 @@ where
     S: SerializeMap,
 {
     #[cfg(all(tracing_unstable, feature = "valuable"))]
+    #[cfg_attr(docsrs, doc(cfg(all(tracing_unstable, feature = "valuable"))))]
     fn record_value(&mut self, field: &Field, value: valuable_crate::Value<'_>) {
         if self.state.is_ok() {
             self.state = self
@@ -436,6 +437,7 @@ where
     S: SerializeStruct,
 {
     #[cfg(all(tracing_unstable, feature = "valuable"))]
+    #[cfg_attr(docsrs, doc(cfg(all(tracing_unstable, feature = "valuable"))))]
     fn record_value(&mut self, field: &Field, value: valuable_crate::Value<'_>) {
         if self.state.is_ok() {
             self.state = self

tracing-subscriber/src/lib.rs

@@ -103,6 +103,37 @@
 //! tracing-subscriber = { version = "0.3", default-features = false, features = ["alloc"] }
 //! ```
 //!
+//! ### Unstable Features
+//!
+//! These feature flags enable **unstable** features. The public API may break in 0.1.x
+//! releases. To enable these features, the `--cfg tracing_unstable` must be passed to
+//! `rustc` when compiling.
+//!
+//! The following unstable feature flags are currently available:
+//!
+//! * `valuable`: Enables support for serializing values recorded using the
+//!   [`valuable`] crate as structured JSON in the [`format::Json`] formatter.
+//!
+//! #### Enabling Unstable Features
+//!
+//! The easiest way to set the `tracing_unstable` cfg is to use the `RUSTFLAGS`
+//! env variable when running `cargo` commands:
+//!
+//! ```shell
+//! RUSTFLAGS="--cfg tracing_unstable" cargo build
+//! ```
+//! Alternatively, the following can be added to the `.cargo/config` file in a
+//! project to automatically enable the cfg flag for that project:
+//!
+//! ```toml
+//! [build]
+//! rustflags = ["--cfg", "tracing_unstable"]
+//! ```
+//!
+//! [feature flags]: https://doc.rust-lang.org/cargo/reference/manifest.html#the-features-section
+//! [`valuable`]: https://crates.io/crates/valuable
+//! [`format::Json`]: crate::fmt::format::Json
+//!
 //! ## Supported Rust Versions
 //!
 //! Tracing is built against the latest stable release. The minimum supported