add note leading into atomics

Author: IanButterworth

doc/src/manual/multi-threading.md

@@ -284,6 +284,9 @@ Note that we do not use buffers based on the `threadid()` i.e. `buffers = zeros(
 actually change thread at yield points, known as task migration (docs??), which would introduce data races for such a
 buffering approach.
 
+Another option, which would not be performant for a naive sum like this, is the use of atomic operations on variables shared
+across all threads.
+
 ## Atomic Operations
 
 Julia supports accessing and modifying values *atomically*, that is, in a thread-safe way to avoid