sync: document that Mutex::const_new() is not instrumented

The mutex created by `Mutex::const_new()` does not contain the tracing
instrumentation that drives `tokio-console`.

This change adds documentation to inform the user of this.

tokio/src/sync/mutex.rs

@@ -371,13 +371,21 @@ impl<T: ?Sized> Mutex<T> {
 
     /// Creates a new lock in an unlocked state ready for use.
     ///
+    /// When using the `tracing` [unstable feature], a `Mutex` created with
+    /// `const_new` will not be instrumented. As such, it will not be visible
+    /// in [`tokio-console`] Instead, [`Mutex::new`] should be used to create
+    /// an instrumented object if that is needed.
+    ///
     /// # Examples
     ///
     /// ```
     /// use tokio::sync::Mutex;
     ///
     /// static LOCK: Mutex<i32> = Mutex::const_new(5);
     /// ```
+    ///
+    /// [`tokio-console`]: https://github.com/tokio-rs/console
+    /// [unstable feature]: crate#unstable-features
     #[cfg(not(all(loom, test)))]
     pub const fn const_new(t: T) -> Self
     where