Add some documentation about *when* broadcasts run

rayon-core/src/thread_pool/mod.rs

@@ -114,6 +114,13 @@ impl ThreadPool {
  /// `join`, `scope`, or parallel iterators will then operate within that
  /// threadpool.
  ///
+ /// Broadcasts are executed on each thread after they have exhausted their
+ /// local work queue, before they attempt work-stealing from other threads.
+ /// The goal of that strategy is to run everywhere in a timely manner
+ /// *without* being too disruptive to current work. There may be alternative
+ /// broadcast styles added in the future for more or less aggressive
+ /// injection, if the need arises.
+ ///
  /// # Warning: thread-local data
  ///
  /// Because `op` is executing within the Rayon thread-pool,