tracing: fix a typo and adds a note about the supported field syntax (tokio-rs#1232)

The `instrument` macro does not explain the syntax that can be used and
the main tracing docs only say that this syntax is valid for `span` and
`event` macros.

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

Author: 2 people

tracing-attributes/src/lib.rs

@@ -113,6 +113,8 @@ mod expand;
 /// for each field. The name of the field must be a single valid Rust
 /// identifier, nested (dotted) field names are not supported.
 ///
+/// This supports the same [field syntax] as the `span!` and `event!` macros.
+///
 /// Note that overlap between the names of fields and (non-skipped) arguments
 /// will result in a compile error.
 ///

tracing/src/lib.rs

@@ -208,8 +208,8 @@
 //!
 //! ## Using the Macros
 //!
-//! The [`span!`] and [`event!`] macros use fairly similar syntax, with some
-//! exceptions.
+//! The [`span!`] and [`event!`] macros as well as the `#[instrument]` attribute
+//! use fairly similar syntax, with some exceptions.
 //!
 //! ### Configuring Attributes
 //!