tracing: clarify span enter docs warning (#2010)

## Motivation

It took me several reads to understand why holding a guard over an await
was bad because my brain is naturally synchronous and could only think
about `my_async_function` and `some_other_async_function`

## Solution

Rather than just iterating "this is bad" use that opportunity to
introduce why. Remind the user that this is being executed concurrently
to other tasks

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

Author: liuhaozhan

tracing/src/span.rs

@@ -605,9 +605,13 @@ impl Span {
     /// async fn my_async_function() {
     ///     let span = info_span!("my_async_function");
     ///
-    ///     // THIS WILL RESULT IN INCORRECT TRACES
+    ///     // WARNING: This span will remain entered until this
+    ///     // guard is dropped...
     ///     let _enter = span.enter();
-    ///     some_other_async_function().await;
+    ///     // ...but the `await` keyword may yield, causing the
+    ///     // runtime to switch to another task, while remaining in
+    ///     // this span!
+    ///     some_other_async_function().await
     ///
     ///     // ...
     /// }