docs: document span.in_scope() at top-level (#1344)

## Motivation

`span.in_scope()` had a link def in the main tracing docs which was
unused, this function is quite handy to know about and I almost
re-implemented something similar to it.

I almost reimplemented this as a macro.


## Solution

Document it at top-level!

Co-authored-by: Eliza Weisman <[email protected]>

tracing/src/lib.rs

@@ -173,6 +173,25 @@
 //! # fn main() {}
 //! ```
 //!
+//! For functions which don't have built-in tracing support and can't have
+//! the `#[instrument]` attribute applied (such as from an external crate,
+//! the [`Span` struct][`Span`] has a [`in_scope()` method][`in_scope`]
+//! which can be used to easily wrap synchonous code in a span.
+//!
+//! For example:
+//! ```rust
+//! use tracing::info_span;
+//!
+//! # fn doc() -> Result<(), ()> {
+//! # mod serde_json {
+//! #    pub(crate) fn from_slice(buf: &[u8]) -> Result<(), ()> { Ok(()) }
+//! # }
+//! # let buf: [u8; 0] = [];
+//! let json = info_span!("json.parse").in_scope(|| serde_json::from_slice(&buf))?;
+//! # let _ = json; // suppress unused variable warning
+//! # Ok(())
+//! # }
+//! ```
 //!
 //! You can find more examples showing how to use this crate [here][examples].
 //!