doc: add additional Mutex example (#2019)

tokio/src/sync/mutex.rs

@@ -26,6 +26,51 @@
 //! }
 //! ```
 //!
+//! Another example
+//! ```rust,no_run
+//! #![warn(rust_2018_idioms)]
+//!
+//! use tokio::sync::Mutex;
+//! use std::sync::Arc;
+//!
+//!
+//! #[tokio::main]
+//! async fn main() {
+//!    let count = Arc::new(Mutex::new(0));
+//!
+//!    for _ in 0..5 {
+//!        let my_count = Arc::clone(&count);
+//!        tokio::spawn(async move {
+//!            for _ in 0..10 {
+//!                let mut lock = my_count.lock().await;
+//!                *lock += 1;
+//!                println!("{}", lock);
+//!            }
+//!        });
+//!    }
+//!
+//!    loop {
+//!        if *count.lock().await >= 50 {
+//!            break;
+//!        }
+//!    }
+//!   println!("Count hit 50.");
+//! }
+//! ```
+//! There are a few things of note here to pay attention to in this example.
+//! 1. The mutex is wrapped in an [`std::sync::Arc`] to allow it to be shared across threads.
+//! 2. Each spawned task obtains a lock and releases it on every iteration.
+//! 3. Mutation of the data the Mutex is protecting is done by de-referencing the the obtained lock
+//!    as seen on lines 23 and 30.
+//!
+//! Tokio's Mutex works in a simple FIFO (first in, first out) style where as requests for a lock are
+//! made Tokio will queue them up and provide a lock when it is that requester's turn. In that way
+//! the Mutex is "fair" and predictable in how it distributes the locks to inner data. This is why
+//! the output of this program is an in-order count to 50. Locks are released and reacquired
+//! after every iteration, so basically, each thread goes to the back of the line after it increments
+//! the value once. Also, since there is only a single valid lock at any given time there is no
+//! possibility of a race condition when mutating the inner value.
+//!
 //! Note that in contrast to `std::sync::Mutex`, this implementation does not
 //! poison the mutex when a thread holding the `MutexGuard` panics. In such a
 //! case, the mutex will be unlocked. If the panic is caught, this might leave