sync: document FIFO behavior of tokio::sync::Mutex (#6279)

Signed-off-by: azuredream <zhaozixuan67@gmail.com>

tokio/src/sync/mutex.rs

@@ -17,6 +17,10 @@ use std::{fmt, mem, ptr};
 /// differences: [`lock`] is an async method so does not block, and the lock
 /// guard is designed to be held across `.await` points.
 ///
+/// Tokio's Mutex operates on a guaranteed FIFO basis.
+/// This means that the order in which tasks call the [`lock`] method is
+/// the exact order in which they will acquire the lock.
+///
 /// # Which kind of mutex should you use?
 ///
 /// Contrary to popular belief, it is ok and often preferred to use the ordinary