Document DisplayManager atomicity with AChoreographer

Very early in Android 12's development, AChoreographer's refresh rate
callbacks were updated to dispatch from DisplayManager rather than from
SurfaceFlinger. Now that we've written a test enforcing this behavior,
update the documentation so that API users can depend on more reliable
behavior.

Bug: 168708518
Test: builds
Change-Id: I4f28c7a8d7b92b45dc2b89ba43595a6f0434d09d
diff --git a/include/android/choreographer.h b/include/android/choreographer.h
index 8039bb0..cc5420e 100644
--- a/include/android/choreographer.h
+++ b/include/android/choreographer.h
@@ -127,6 +127,10 @@
  * to listen directly to DisplayManager.DisplayListener#onDisplayChanged events
  * instead.
  *
+ * As of API level 31, this api is guaranteed to have a consistent view with DisplayManager;
+ * Display#getRefreshRate is guaranteed to not return a stale refresh rate when invoked from this
+ * callback.
+ *
  * Available since API level 30.
  */
 void AChoreographer_registerRefreshRateCallback(AChoreographer* choreographer,