Docs: Use non-SeqCst in module example of atomics

I done this for this reasons:
1. The example now shows that there is more Orderings than just SeqCst.
2. People who would copy from example would now have more suitable orderings for the job.
3. SeqCst is both much harder to reason about and not needed in most situations.

IMHO, we should encourage people to think and use memory orderings that is suitable to task instead of blindly defaulting to SeqCst.

Author: AngelicosPhosphoros

library/core/src/sync/atomic.rs

@@ -138,7 +138,7 @@
 //!
 //! In general, *all* atomic accesses on read-only memory are Undefined Behavior. For instance, attempting
 //! to do a `compare_exchange` that will definitely fail (making it conceptually a read-only
-//! operation) can still cause a page fault if the underlying memory page is mapped read-only. Since
+//! operation) can still cause a segmentation fault if the underlying memory page is mapped read-only. Since
 //! atomic `load`s might be implemented using compare-exchange operations, even a `load` can fault
 //! on read-only memory.
 //!
@@ -181,12 +181,13 @@
 //!     let spinlock = Arc::new(AtomicUsize::new(1));
 //!
 //!     let spinlock_clone = Arc::clone(&spinlock);
+//!
 //!     let thread = thread::spawn(move|| {
-//!         spinlock_clone.store(0, Ordering::SeqCst);
+//!         spinlock_clone.store(0, Ordering::Release);
 //!     });
 //!
 //!     // Wait for the other thread to release the lock
-//!     while spinlock.load(Ordering::SeqCst) != 0 {
+//!     while spinlock.load(Ordering::Acquire) != 0 {
 //!         hint::spin_loop();
 //!     }
 //!
@@ -203,7 +204,11 @@
 //!
 //! static GLOBAL_THREAD_COUNT: AtomicUsize = AtomicUsize::new(0);
 //!
-//! let old_thread_count = GLOBAL_THREAD_COUNT.fetch_add(1, Ordering::SeqCst);
+//! // Note that Relaxed ordering doesn't synchronize anything
+//! // except the global thread counter itself.
+//! let old_thread_count = GLOBAL_THREAD_COUNT.fetch_add(1, Ordering::Relaxed);
+//! // Note that this number may not be true at the moment of printing
+//! // because some other thread may have changed static value already.
 //! println!("live threads: {}", old_thread_count + 1);
 //! ```