Rollup merge of rust-lang#49916 - llogiq:doc-atomic-fetch-update, r=kennytm

improve Atomic*::fetch_update docs

This clarifies that fetch_update *always* returns the previous value, either as `Ok(_)` or `Err(_)`, depending on whether the supplied update function returned `Some(_)` or `None`.

Author: kennytm

src/libcore/sync/atomic.rs

@@ -1425,8 +1425,8 @@ assert_eq!(foo.load(Ordering::SeqCst), 0b011110);
 
             doc_comment! {
                 concat!("Fetches the value, and applies a function to it that returns an optional
-new value. Returns a `Result` (`Ok(_)` if the function returned `Some(_)`, else `Err(_)`) of the
-previous value.
+new value. Returns a `Result` of `Ok(previous_value)` if the function returned `Some(_)`, else
+`Err(previous_value)`.
 
 Note: This may call the function multiple times if the value has been changed from other threads in
 the meantime, as long as the function returns `Some(_)`, but the function will have been applied