Rollup merge of rust-lang#34046 - Vtec234:fix-atomic-doc, r=steveklabnik

Fix wrong statement in compare_exchange doc

The documentation for `core::sync::atomic::AtomicSomething::compare_exchange` contains a wrong, or imprecise, statement about the return value. It goes:

The return value is a result indicating whether the new value was written and containing
the previous value. On success this value is guaranteed to be equal to `new`.

In the second sentence, `this value` is gramatically understood as referring to `return value` from the first sentence. Due to how CAS works, the returned value is always what was in the atomic variable _before_ the operation occurred, not what was written into it during the operation. Hence, the fixed doc should say:

The return value is a result indicating whether the new value was written and containing
the previous value. On success this value is guaranteed to be equal to `current`.

This version is confirmed by the runnable examples in variants of `AtomicSomething`, e.g.

    assert_eq!(some_bool.compare_exchange(true, false, Ordering::Acquire, Ordering::Relaxed),
    Ok(true));

where the returned value is `Ok(current)`. This PR fixes all occurrences of this bug I could find.

An alternative solution would be to modify the second sentence so that it refers to the value _written_ into the Atomic rather than what was there before, in which case it would be correct. Example alternative formulation:

On success the value written into the `bool`/`usize`/`whatever` is guaranteed to be equal to `new`.

r? @steveklabnik

Author: Manishearth

src/libcore/sync/atomic.rs

@@ -289,7 +289,7 @@ impl AtomicBool {
     /// Stores a value into the `bool` if the current value is the same as the `current` value.
     ///
     /// The return value is a result indicating whether the new value was written and containing
-    /// the previous value. On success this value is guaranteed to be equal to `new`.
+    /// the previous value. On success this value is guaranteed to be equal to `current`.
     ///
     /// `compare_exchange` takes two `Ordering` arguments to describe the memory ordering of this
     /// operation. The first describes the required ordering if the operation succeeds while the
@@ -633,7 +633,7 @@ impl<T> AtomicPtr<T> {
     /// Stores a value into the pointer if the current value is the same as the `current` value.
     ///
     /// The return value is a result indicating whether the new value was written and containing
-    /// the previous value. On success this value is guaranteed to be equal to `new`.
+    /// the previous value. On success this value is guaranteed to be equal to `current`.
     ///
     /// `compare_exchange` takes two `Ordering` arguments to describe the memory ordering of this
     /// operation. The first describes the required ordering if the operation succeeds while the
@@ -886,7 +886,7 @@ macro_rules! atomic_int {
             ///
             /// The return value is a result indicating whether the new value was written and
             /// containing the previous value. On success this value is guaranteed to be equal to
-            /// `new`.
+            /// `current`.
             ///
             /// `compare_exchange` takes two `Ordering` arguments to describe the memory ordering of
             /// this operation. The first describes the required ordering if the operation succeeds