clarify that the map-reduce example relies on static data

src/std_misc/threads/testcase_mapreduce.md

@@ -17,9 +17,11 @@ its tiny block of digits, and subsequently we will sum the intermediate sums pro
 thread.
 
 Note that, although we're passing references across thread boundaries, Rust understands that we're
-only passing read-only references, and that thus no unsafety or data races can occur. Because
-we're `move`-ing the data segments into the thread, Rust will also ensure the data is kept alive
-until the threads exit, so no dangling pointers occur.
+only passing read-only references, and that thus no unsafety or data races can occur. Also because
+the references we're passing have `'static` lifetimes, Rust understands that our data won't be
+destroyed while these threads are still running. (When you need to share non-`static` data between
+threads, you can use a smart pointer like `Arc` to keep the data alive and avoid non-`static`
+lifetimes.)
 
 ```rust,editable
 use std::thread;