message: Clarify timing model.
diff: @@ -354,7 +354,7 @@ impl Bencher {
  /// and coordinate with multiple threads).
  ///
  /// # Timing model
  - /// Can be customized and depends on the implementation of `routine`.
  + /// Custom, the timing model is whatever is returned as the Duration from `routine`.
  ///
  /// # Example
  /// ```rust
  